Tag: property

content-visibility: the new CSS property that boosts your rendering performance

Una Kravets and Vladimir Levin:

[…] you can use another CSS property called content-visibility to apply the needed containment automatically. content-visibility ensures that you get the largest performance gains the browser can provide with minimal effort from you as a developer.

The content-visibility property accepts several values, but auto is the one that provides immediate performance improvements.

The perf benefits seems pretty big:

In our example, we see a boost from a 232ms rendering time to a 30ms rendering time. That’s a 7x performance boost.

It’s manual work though. You have to “section” large vertical chunks of the page yourself, apply content-visibility: auto; to them, then take a stab at about how tall they are, something like contain-intrinsic-size: 1000px;. That part seems super weird to me. Just guess at a height? What if I’m wrong? Can I hurt performance? Can (or should) I change that value at different viewports if the height difference between small and large screens is drastic?

Seems like you’d have to be a pretty skilled perf nerd to get this right, and know how to look at and compare rendering profiles in DevTools. All the more proof that web perf is its own vocation.

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Using @property for CSS Custom Properties

Una Kravetz digs into how Chrome now allows you to declare CSS custom properties directly from CSS with more information than just a string.

So rather than something like this:

html {   --stop: 50%; }

…can be declared with more details like this:

@property --stop {   syntax: "<percentage>";   initial-value: 50%;   inherits: false; }

The browser then knows this specific custom property is a percentage rather than a string. It can be other useful stuff like <integer> and <color>. Now that we have a way to communicate this sort of information to the browser, we get some new abilities, like being able to transition between two values.

While playing around, I noticed you have to very specifically call out the property to be transitioned (because a catch-all transition won’t do it). Try hovering on this demo, which is a re-creation of what Una did in the post:

Note that I’m animating the color stop’s position (which is a percentage), but I’m also trying to animate the color, which still does not work. I assumed it would with this new feature. I know people have been confused about the lack of being able to animate gradients for a long time. (See Ana Tudor’s article.)

You can always re-declare the properties somewhere at a high-level to “support” browsers that can’t read custom properties. Feels like a funny time to be talking about that. Safari seems to signal strong interest in this Houdini-based stuff, but hasn’t yet. Firefox? Eeesh, I dunno. Best we know is they labeled it as “Worth Prototyping” before all the layoffs.

This will also help with a the weird fallback issue with CSS custom properties that we mentioned in the newsletter:

As with any other custom property, you can get (using var) or set (write/rewrite) values, but with Houdini custom properties, if you set a falsey value when overriding it, the CSS rendering engine will send the initial value (its fallback value) instead of ignoring the line.

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Can you get valid CSS property values from the browser?

I had someone write in with this very legit question. Lea just blogged about how you can get valid CSS properties themselves from the browser. That’s like this.

That gives you, for example, the fact that cursor is a thing. But then how do you know what valid values are for cursor? We know from documentation that there are values like auto, none, help, context-menu, pointer, progress, wait, and many more.

But where does that list come from? Well, there is a list right in the spec so that’s helpful. But that doesn’t guarantee the complete list of values that any given browser actually supports. There could be cursor: skull-and-crossbones and we wouldn’t even know!

We can test by applying it to an element and looking in DevTools:


But unless we launch a huge dictionary attack against that value, we don’t actually know what values it directly in-browser. Maybe Houdini will help somehow in browsers getting better at CSS introspection?

You can also use the CSS object to run tests like CSS.supports(property, value):


You’d think we could have like CSS.validValues("text-decoration-thickness") and get like ["<length>", "<percentage>", "auto", "from-font"] or the like, but alas, not a thing.

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Using Custom Property “Stacks” to Tame the Cascade

Since the inception of CSS in 1994, the cascade and inheritance have defined how we design on the web. Both are powerful features but, as authors, we’ve had very little control over how they interact. Selector specificity and source order provide some minimal “layering” control, without a lot of nuance — and inheritance requires an unbroken lineage. Now, CSS Custom Properties allow us to manage and control both cascade and inheritance in new ways.

I want to show you how I’ve used Custom Property “stacks” to solve some of the common issues people face in the cascade: from scoped component styles, to more explicit layering of intents.

A quick intro to Custom Properties

The same way browsers have defined new properties using a vendor prefix like -webkit- or -moz-, we can define our own Custom Properties with an “empty” -- prefix. Like variables in Sass or JavaScript, we can use them to name, store, and retrieve values — but like other properties in CSS, they cascade and inherit with the DOM.

/* Define a custom property */ html {   --brand-color: rebeccapurple; }

In order to access those captured values, we use the var() function. It has two parts: first the name of our custom property, and then a fallback in case that property is undefined:

button {   /* use the --brand-color if available, or fall back to deeppink */   background: var(--brand-color, deeppink); }

This is not a support fallback for old browsers. If a browser doesn’t understand custom properties, it will ignore the entire var() declaration. Instead, this is a built-in way of handling undefined variables, similar to a font stack defining fallback font families when one is unavailable. If we don’t provide a fallback, the default is unset.

Building variable “stacks”

This ability to define a fallback is similar to “font stacks” used on the font-family property. If the first family is unavailable, the second will be used, and so on. The var() function only accepts a single fallback, but we can nest var() functions to create custom-property fallback “stacks” of any size:

button {   /* try Consolas, then Menlo, then Monaco, and finally monospace */   font-family: Consolas, Menlo, Monaco, monospace;    /* try --state, then --button-color, then --brand-color, and finally deeppink */   background: var(--state, var(--button-color, var(--brand-color, deeppink))); }

If that nested syntax for stacked properties looks bulky, you can use a pre-processor like Sass to make it more compact.

That single-fallback limitation is required to support fallbacks with a comma inside them — like font stacks or layered background images:

html {   /* The fallback value is "Helvetica, Arial, sans-serif" */   font-family: var(--my-font, Helvetica, Arial, sans-serif); }

Defining “scope”

CSS selectors allow us to drill down into the HTML DOM tree, and style elements anywhere on the page, or elements in a particular nested context.

/* all links */ a { color: slateblue; }  /* only links inside a section */ section a { color: rebeccapurple; }  /* only links inside an article */ article a { color: deeppink; }

That’s useful, but it doesn’t capture the reality of “modular” object-oriented or component-driven styles. We might have multiple articles and asides, nested in various configurations. We need a way to clarify which context, or scope, should take precedence when they overlap.

Proximity scopes

Let’s say we have a .light theme and a .dark theme. We can use those classes on the root <html> element to define a page-wide default, but we can also apply them to specific components, nested in various ways:

Each time we apply one of our color-mode classes, the background and color properties are reset, then inherited by nested headings and paragraphs. In our main context, colors inherit from the .light class, while the nested heading and paragraph inherit from the .dark class. Inheritance is based on direct lineage, so the nearest ancestor with a defined value will take precedence. We call that proximity.

Proximity matters for inheritance, but it has no impact on selectors, which rely on specificity. That becomes a problem if we want to style something inside the dark or light containers.

Here I’ve attempted to define both light and dark button variants. Light mode buttons should be rebeccapurple with white text so they stand out, and dark mode buttons should be plum with black text. We’re selecting the buttons directly based on a light and dark context, but it doesn’t work:

Some of the buttons are in both contexts, with both .light and .dark ancestors. What we want in that case is for the closest theme to take over (inheritance proximity behavior), but what we get instead is the second selector overriding the first (cascade behavior). Since the two selectors have the same specificity, source order determines the winner.

Custom Properties and proximity

What we need here is a way to inherit these properties from the theme, but only apply them to specific children. Custom Properties make that possible! We can define values on the light and dark containers, while only using their inherited values on nested elements, like our buttons.

We’ll start by setting up the buttons to use custom properties, with a fallback “default” value, in case those properties are undefined:

button {   background: var(--btn-color, rebeccapurple);   color: var(--btn-contrast, white); }

Now we can set those values based on context, and they will scope to the appropriate ancestor based on proximity and inheritance:

.dark {   --btn-color: plum;   --btn-contrast: black; }  .light {   --btn-color: rebeccapurple;   --btn-contrast: white; }

As an added bonus, we’re using less code overall, and one unified button definition:

I think of this as creating an API of available parameters for the button component. Sara Soueidan and Lea Verou have both covered this well in recent articles.

Component ownership

Sometimes proximity isn’t enough to define scope. When JavaScript frameworks generate “scoped styles” they are establishing specific object-element ownership. A “tab layout” component owns the tabs themselves, but not the content behind each tab. This is also what the BEM convention attempts to capture in complex .block__element class names.

Nicole Sullivan coined the term “donut scope” to talk about this problem back in 2011. While I’m sure she has more recent thoughts on the issue, the fundamental problem hasn’t changed. Selectors and specificity are great for describing how we build detailed styles over top of broad patterns, but they don’t convey a clear sense of ownership.

We can use custom property stacks to help solve this problem. We’ll start by creating “global” properties on the <html> element that are for our default colors:

html {   --background--global: white;   --color--global: black;   --btn-color--global: rebeccapurple;   --btn-contrast--global: white; }

That default global theme is now available anywhere we want to refer to it. We’ll do that with a data-theme attribute that applies our foreground and background colors. We want the global values to provide a default fallback, but we also want the option to override with a specific theme. That’s where “stacks” come in:

[data-theme] {   /* If there's no component value, use the global value */   background: var(--background--component, var(--background--global));   color: var(--color--component, var(--color--global)); }

Now we can define an inverted component by setting the *--component properties as a reverse of the global properties:

[data-theme='invert'] {   --background--component: var(--color--global);   --color--component: var(--background--global); }

But we don’t want those settings to inherit beyond the donut of ownership, so we reset those values to initial (undefined) on every theme. We’ll want to do this at a lower specificity, or earlier in the source order, so it provides a default that each theme can override:

[data-theme] {   --background--component: initial;   --color--component: initial; }

The initial keyword has a special meaning when used on custom properties, reverting them to a Guaranteed-Invalid state. That means rather than being passed along to set background: initial or color: initial, the custom property becomes undefined, and we fallback to the next value in our stack, the global settings.

We can do the same thing with our buttons, and then make sure to apply data-theme to each component. If no specific theme is given, each component will default to the global theme:

Defining “origins”

The CSS cascade is a series of filtering layers used to determine what value should take precedence when multiple values are defined on the same property. We most often interact with the specificity layers, or the final layering based on source-order — but the first layer of cascade is the “origin” of a style. The origin describes where a style came from — often the browser (defaults), the user (preferences), or the author (that’s us).

By default, author styles override user preferences, which override browser defaults. That changes when anyone applies `!important` to a style, and the origins reverse: browser `!important` styles have the highest origin, then important user preferences, then our author important styles, above all the normal layers. There are a few additional origins, but we won’t go into them here.

When we create custom property “stacks,” we’re building a very similar behavior. If we wanted to represent existing origins as a stack of custom properties, it would look something like this:

.origins-as-custom-properties {   color: var(--browser-important, var(--user-important, var(--author-important, var(--author, var(--user, var(--browser)))))); }

Those layers already exist, so there’s no reason to recreate them. But we’re doing something very similar when we layer our “global” and “component” styles above — creating a “component” origin layer that overrides our “global” layer. That same approach can be used to solve various layering issues in CSS, which can’t always be described by specificity:

  • Override » Component » Theme » Default
  • Theme » Design system or framework
  • State » Type » Default

Let’s look at some buttons again. We’ll need a default button style, a disabled state, and various button “types,” like danger, primary and secondary. We wan’t the disabled state to always override the type variations, but selectors don’t capture that distinction:

But we can define a stack that provides both “type” and “state” layers in the order that we want them prioritized:

button {   background: var(--btn-state, var(--btn-type, var(--btn-default))); }

Now when we set both variables, the state will always take precedence:

I’ve used this technique to create a Cascading Colors framework that allows custom theming based on layering:

  • Pre-defined theme attributes in the HTML
  • User color preferences
  • Light and dark modes
  • Global theme defaults

Mix and match

These approaches can be taken to an extreme, but most day-to-day use-cases can be handled with two or three values in a stack, often using a combination of the techniques above:

  • A variable stack to define the layers
  • Inheritance to set them based on proximity and scope
  • Careful application of the `initial` value to remove nested elements from a scope

We’ve been using these custom property “stacks” on our projects at OddBird. We’re still discovering as we go, but they’ve already been helpful in solving problems that were difficult using only selectors and specificity. With custom properties, we don’t have to fight the cascade or inheritance. We can capture and leverage them, as-intended, with more control over how they should apply in each instance. To me, that’s a big win for CSS — especially when developing style frameworks, tools, and systems.

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Let’s Take a Deep Dive Into the CSS Contain Property

Compared to the past, modern browsers have become really efficient at rendering the tangled web of HTML, CSS, and JavaScript code a typical webpage provides. It takes a mere milliseconds to render the code we give it into something people can use.

What could we, as front-end developers, do to actually help the browser be even faster at rendering? There are the usual best practices that are so easy to forget with our modern tooling — especially in cases where we may not have as much control over generated code. We could keep our CSS under control, for instance, with fewer and simpler selectors. We could keep our HTML under control; keep the tree flatter with fewer nodes, and especially fewer children. We could keep our JavaScript under control; while being careful with our HTML and CSS manipulations.

Actually, modern frameworks such as Vue and React do help out a good bit with that last part.

I would like to explore a CSS property that we could use to help the browser figure out what calculations it can reduce in priority or maybe even skip altogether.

This property is called contain. Here is how MDN defines this property:

The contain CSS property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited area of the DOM and not the entire page, leading to obvious performance benefits.

A simple way to look at what this property provides is that we can give hints to the browser about the relationships of the various elements on the page. Not necessarily smaller elements, such as paragraphs or links, but larger groups such as sections or articles. Essentially, we’re talking about container elements that hold content — even content that can be dynamic in nature. Think of a typical SPA where dynamic content is being inserted and removed throughout the page, often independent of other content on the page.

A browser cannot predict the future of layout changes to the webpage that can happen from JavaScript inserting and removing content on the page. Even simple things as inserting a class name to an element, animating a DOM element, or just getting the dimensions of an element can cause a reflow and repaint of the page. Such things can be expensive and should be avoided, or at least be reduced as much as possible.

Developers can sort of predict the future because they’ll know about possible future changes based on the UX of the page design, such as when the user clicks on a button it will call for data to be inserted in a div located somewhere in the current view. We know that’s a possibility, but the browser does not. We also know that there’s a distinct possibility that inserting data in that div will not change anything visually, or otherwise, for other elements on the page.

Browser developers have spent a good amount of time optimizing the browser to handle such situations. There are various ways of helping the browser be more efficient in such situations, but more direct hints would be helpful. The contain property gives us a way to provide these hints.

The various ways to contain

The contain property has three values that can be used individually or in combination with one another: size, layout, and paint. It also has two shorthand values for common combinations: strict and content. Let’s cover the basics of each.

Please keep in mind that there are a number of rules and edge cases for each of these that are covered in the spec. I would imagine these will not be of much concern in most situations. Yet, if you get an undesired result, then a quick look at the spec might be handy.

There is also a style containment type in the spec that this article will not cover. The reason being that the style containment type is considered of little value at this time and is currently at-risk of being removed from the spec.

Size containment

size containment is actually a simple one to explain. When a container with this containment is involved in the layout calculations, the browser can skip quite a bit because it ignores the children of that container. It is expected the container will have a set height and width; otherwise, it collapses, and that is the only thing considered in layout of the page. It is treated as if it has no content whatsoever.

Consider that descendants can affect their container in terms of size, depending on the styles of the container. This has to be considered when calculating layout; with size containment, it most likely will not be considered. Once the container’s size has been resolved in relation to the page, then the layout of its descendants will be calculated.

size containment doesn’t really provide much in the way of optimizations. It is usually combined with one of the other values.

Although, one benefit it could provide is helping with JavaScript that alters the descendants of the container based on the size of the container, such as a container query type situation. In some circumstances, altering descendants based on the container’s size can cause the container to change size after the change was done to the descendants. Since a change in the container’s size can trigger another change in the descendants you could end up with a loop of changes. size containment can help prevent that loop.

Here’s a totally contrived example of this resizing loop concept:

In this example, clicking the start button will cause the red box to start growing, based on the size of the purple parent box, plus five pixels. As the purple box adjusts in size, a resize observer tells the red square to again resize based on the size of the parent. This causes the parent to resize again and so on. The code stops this process once the parent gets above 300 pixels to prevent the infinite loop.

The reset button, of course, puts everything back into place.

Clicking the checkbox “set size containment” sets different dimensions and the size containment on the purple box. Now when you click on the start button, the red box will resize itself based on the width of the purple box. True, it overflows the parent, but the point is that it only resizes the one time and stops; there’s no longer a loop.

If you click on the resize container button, the purple box will grow wider. After the delay, the red box will resize itself accordingly. Clicking the button again returns the purple box back to its original size and then the red box will resize again.

While it is possible to accomplish this behavior without use of the containment, you will miss out on the benefits. If this is a situation that can happen often in the page the containment helps out with page layout calculations. When the descendants change internal to the containment, the rest of the page behaves as if the changes never happened.

Layout containment

layout containment tells the browser that external elements neither affect the internal layout of the container element, nor does the internal layout of the container element affect external elements. So when the browser does layout calculations, it can assume that the various elements that have the layout containment won’t affect other elements. This can lower the amount of calculations that need to be done.

Another benefit is that related calculations could be delayed or lowered in priority if the container is off-screen or obscured. An example the spec provides is:

[…] for example, if the containing box is near the end of a block container, and you’re viewing the beginning of the block container

The container with layout containment becomes a containing box for absolute or fixed position descendants. This would be the same as applying a relative position to the container. So, keep that in mind how the container’s descendants may be affected when applying this type of containment.

On a similar note, the container gets a new stacking context, so z-index can be used the same as if a relative, absolute, or fixed position was applied. Although, setting the top, right, bottom, or left properties has no effect on the container.

Here’s a simple example of this:

Click the box and layout containment is toggled. When layout containment is applied, the two purple lines, which are absolute positioned, will shift to inside the purple box. This is because the purple box becomes a containing block with the containment. Another thing to note is that the container is now stacked on top of the green lines. This is because the container now has a new stacking context and follows those rules accordingly.

Paint containment

paint containment tells the browser none of the children of the container will ever be painted outside the boundaries of the container’s box dimensions. This is similar to placing overflow: hidden; on the container, but with a few differences.

For one, the container gets the same treatment as it does under layout containment: it becomes a containing block with its own stacking context. So, having positioned children inside paint containment will respect the container in terms of placement. If we were to duplicate the layout containment demo above but use paint containment instead, the outcome would be much the same. The difference being that the purple lines would not overflow the container when containment is applied, but would be clipped at the container’s border-box.

Another interesting benefit of paint containment is that the browser can skip that element’s descendants in paint calculations if it can detect that the container itself is not visible within the viewport. If the container is not in the viewport or obscured in some way, then it’s a guarantee that its descendants are not visible as well. As an example, think of a nav menu that normally sits off-screen to the left of the page and it slides in when a button is clicked. When that menu is in its normal state off-screen, the browser just skips trying to paint its contents.

Containments working together

These three containments provide different ways of influencing parts of rendering calculations performed by the browser. size containment tells the browser that this container should not cause positional shifting on the page when its contents change. layout containment tells the browser that this container’s descendants should not cause layout changes in elements outside of its container and vice-versa. paint containment tells the browser that this container’s content will never be painted outside of the container’s dimensions and, if the container is obscured, then don’t bother painting the contents at all.

Since each of these provide different optimizations, it would make sense to combine some of them. The spec actually allows for that. For example, we could use layout and paint together as values of the contain property like this:

.el {   contain: layout paint; }

Since this is such an obvious thing to do, the spec actually provides two shorthand values:

Shorthand Longhand
content layout paint
strict layout paint size

The content value will be the most common to use in a web project with a number of dynamic elements, such as large multiple containers that have content changing over time or from user activity.

The strict value would be useful for containers that have a defined size that will never change, even if the content changes. Once in place, it’ll stay the intended size. A simple example of that is a div that contains third-party external advertising content, at industry-defined dimensions, that has no relation to anything else on the page DOM-wise.

Performance benefits

This part of the article is tough to explain. The problem is that there isn’t much in the way of visuals about the performance benefits. Most of the benefits are behind-the-scenes optimizations that help the browser decide what to do on a layout or paint change.

As an attempt to show the contain property’s performance benefits, I made a simple example that changes the font-size on an element with several children. This sort of change would normally trigger a re-layout, which would also lead to a repaint of the page. The example covers the contain values of none, content, and strict.

The radio buttons change the value of the contain property being applied to the purple box in the center. The “change font-size” button will toggle the font-size of the contents of the purple box by switching classes. Unfortunately, this class change is also a potential trigger for re-layout. If you’re curious, here is a list of situations in JavaScript and then a similar list for CSS that trigger such layout and paint calculations. I bet there’s more than you think.

My totally unscientific process was to select the contain type, start a performance recording in Chome’s developer tools, click the button, wait for the font-size change, then stop the recording after another second or so. I did this three times for each containment type to be able to compare multiple recordings. The numbers for this type of comparison are in the low milliseconds each, but there’s enough of a difference to get a feel for the benefits. The numbers could potentially be quite different in a more real-world situation.

But there are a few things to note other than just the raw numbers.

When looking through the recording, I would find the relevant area in the timeline and focus there to select the task that covers the change. Then I would look at the event log of the task to see the details. The logged events were: recalculate style, layout, update layer tree, paint, and composite layers. Adding the times of all those gives us the total time of the task.

DevTools showing set time at 27.9 milliseconds which is the same as the total time to recalculate styles.
The event log with no containment.

One thing to note for the two containment types is that the paint event was logged twice. I’ll get back to that in a moment.

DevTools showing set time at 13.8 milliseconds which is the same as the total time to recalculate styles.

Completing the task at hand

Here are the total times for the three containment types, three runs each:

Containment Run 1 Run 2 Run 3 Average
none 24 ms 33.8 ms 23.3 ms 27.03 ms
content 13.2 ms 9 ms 9.2 ms 10.47 ms
strict 5.6 ms 18.9 ms 8.5 ms 11 ms

The majority of the time was spent in layout. There were spikes here and there throughout the numbers, but remember that these are unscientific anecdotal results. In fact, the second run of strict containment had a much higher result than the other two runs; I just kept it in because such things do happen in the real world. Perhaps the music I was listening to at the time changed songs during that run, who knows. But you can see that the other two runs were much quicker.

So, by these numbers you can start to see that the contain property helps the browser render more efficiently. Now imagine my one small change being multiplied over the many changes made to the DOM and styling of a typical dynamic web page.

Where things get more interesting is in the details of the paint event.

Layout once, paint twice

Stick with me here. I promise it will make sense.

I’m going to use the demo above as the basis for the following descriptions. If you wish to follow along then go to the full version of the demo and open the DevTools. Note that you have to open up the details of the “frame” and not the “main” timeline once you run the performance tool to see what I’m about to describe.

Showing frame details open and main details closed in DevTools.
Showing frame details open and main details closed in DevTools

I’m actually taking screenshots from the “fullpage” version since DevTools works better with that version. That said, the regular “full” version should give roughly the same idea.

The paint event only fired once in the event log for the task that had no containment at all. Typically, the event didn’t take too long, ranging from 0.2 ms to 3.6 ms. The deeper details is where it gets interesting. In those details, it notes that the area of paint was the entire page. In the event log, if you hover on the paint event, DevTools will even highlight the area of the page that was painted. The dimensions in this case will be whatever the size of your browser viewport happens to be. It will also note the layer root of the paint.

Showing DevTools paint calculation of 0.7 milliseconds.
Paint event details

Note that the page area to the left in the image is highlighted, even outside of the purple box. Over to the right, are the dimensions of the paint to the screen. That’s roughly the size of the viewport in this instance. For a future comparison, note the #document as the layer root.

Keep in mind that browsers have the concept of layers for certain elements to help with painting. Layers are usually for elements that may overlap each other due to a new stacking context. An example of this is the way applying position: relative; and z-index: 1; to an element will cause the browser to create that element as a new layer. The contain property has the same effect.

There is a section in DevTools called “rendering” and it provides various tools to see how the browser renders the page. When selecting the checkbox named “Layer borders” we can see different things based on the containment. When the containment is none then you should see no layers beyond the typical static web page layers. Select content or strict and you can see the purple box get converted to its own layer and the rest of the layers for the page shift accordingly.

Layers with no containment
Layers with containment

It may be hard to notice in the image, but after selecting content containment the purple box becomes its own layer and the page has a shift in layers behind the box. Also notice that in the top image the layer line goes across on top of the box, while in the second image the layer line is below the box.

I mentioned before that both content and strict causes the paint to fire twice. This is because two painting processes are done for two different reasons. In my demo the first event is for the purple box and the second is for the contents of the purple box.

Typically the first event will paint the purple box and report the dimensions of that box as part of the event. The box is now its own layer and enjoys the benefits that applies.

The second event is for the contents of the box since they are scrolling elements. As the spec explains; since the stacking context is guaranteed, scrolling elements can be painted into a single GPU layer. The dimensions reported in the second event is taller, the height of the scrolling elements. Possibly even narrower to make room for the scrollbar.

First paint event with content containment
Second paint event with content containment

Note the difference in dimensions on the right of both of those images. Also, the layer root for both of those events is main.change instead of the #document seen above. The purple box is a main element, so only that element was painted as opposed as to whole document. You can see the box being highlighted as opposed to the whole page.

The benefits of this is that normally when scrolling elements come into view, they have to be painted. Scrolling elements in containment have already been painted and don’t require it again when coming into view. So we get some scrolling optimizations as well.

Again, this can be seen in the demo.

Back to that Rendering tab. This time, check “Scrolling performance issue” instead. When the containment is set to none, Chrome covers the purple box with an overlay that’s labeled “repaints on scroll.”

DevTools showing “repaints on scroll” with no containment

If you wish to see this happen live, check the “Paint flashing” option.

Please note: if flashing colors on the screen may present an issue for you in some way, please consider not checking the “Paint flashing” option. In the example I just described, not much changes on the page, but if one were to leave that checked and visited other sites, then reactions may be different.

With paint flashing enabled, you should see a paint indicator covering all the text within the purple box whenever you scroll inside it. Now change the containment to content or strict and then scroll again. After the first initial paint flash it should never reappear, but the scrollbar does show indications of painting while scrolling.

Paint flashing enabled and scrolling with no containment
Paint flashing and scrolling with content containment

Also notice that the “repaints on scroll” overlay is gone on both forms of containment. In this case, containment has given us not only some performance boost in painting but in scrolling as well.

An interesting accidental discovery

As I was experimenting with the demo above and finding out how the paint and scrolling performance aspects worked, I came across an interesting issue. In one test, I had a simple box in the center of page, but with minimal styling. It was essentially an element that scrolls with lots of text content. I was applying content containment to the container element, but I wasn’t seeing the scrolling performance benefits described above.

The container was flagged with the “repaints on scroll” overlay and the paint flashing was the same as no containment applied, even though I knew for a fact that content containment was being applied to the container. So I started comparing my simple test against the more styled version I discussed above.

I eventually saw that if the background-color of the container is transparent, then the containment scroll performance benefits do not happen.

I ran a similar performance test where I would change the font-size of the contents to trigger the re-layout and repaint. Both tests had roughly the same results, with only difference that the first test had a transparent background-color and the second test had a proper background-color. By the numbers, it looks like the behind-the-scenes calculations are still more performant; only the paint events are different. It appears the element doesn’t become its own layer in the paint calculations with a transparent background-color.

The first test run only had one paint event in the event log. The second test run had the two paint events as I would expect. Without that background color, it seems the browser decides to skip the layer aspect of the containment. I even found that faking transparency by using the same color as the color behind the element works as well. My guess is if the container’s background is transparent then it must rely on whatever is underneath, making it impossible to separate the container to its own paint layer.

I made another version of the test demo that changes the background-color of the container element from transparent to the same color used for the background color of the body. Here are two screenshots showing the differences when using the various options in the Rendering panel in DevTools.

Rendering panel with transparent background-color

You can see the checkboxes that have been selected and the result to the container. Even with a content containment applied, the box has “repaints on scroll” as well as the green overlay showing painting while scrolling.

Rendering panel with background-color applied

In the second image, you can see that the same checkboxes are selected and a different result to the container. The “repaints on scroll” overlay is gone and the green overlay for painting is also gone. You can see the paint overlay on the scrollbar to show it was active.

Conclusion: make sure to apply some form of background color to your container when applying containment to get all the benefits.

Here’s what I used for the test:

This is the bottom of the page

This article has covered the basics of the CSS contain property with its values, benefits, and potential performance gains. There are some excellent benefits to applying this property to certain elements in HTML; which elements need this applied is up to you. At least, that’s what I gather since I’m unaware of any specific guidance. The general idea is apply it to elements that are containers of other elements, especially those with some form of dynamic aspect to them.

Some possible scenarios: grid areas of a CSS grid, elements containing third-party content, and containers that have dynamic content based on user interaction. There shouldn’t be any harm in using the property in these cases, assuming you aren’t trying to contain an element that does, in fact, rely in some way on another element outside that containment.

Browser support is very strong. Safari is the only holdout at this point. You can still use the property regardless because the browser simply skips over that code without error if it doesn’t understand the property or its value.

So, feel free to start containing your stuff!

The post Let’s Take a Deep Dive Into the CSS Contain Property appeared first on CSS-Tricks.


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The @property is totally new to me, but I see it’s headed to Chrome, so I suppose it’s good to know about!

There is a draft spec and an “intent to ship” document. The code from that document shows:

@property --my-property {   syntax: "<color>";   initial-value: green;   inherits: false; }

That it’s the CSS exact-equivalent to a   CSS.registerProperty(), the JavaScript syntax for declaring CSS custom properties, also a new thing (under the Houdini umbrella, it seems).

It looks like you declare these not within a selector block, but outside (like a @media query), and once you have, you haven’t actually created a new custom property yet, you’ve just registered the fact that you probably will later. When you actually go to create/use the custom property, you create it within a selector block like you already do now.

The “commonly cited use-case” is pretty darn cool. Right now, this isn’t possible in CSS:

.el {   background: linear-gradient(white, black);   /* this transition won't work */   transition: 1s; } .el:hover {   background: linear-gradient(red, black); }

You might think the white in that gradient will fade to red with that transition, but no, that’s not transition-able in that way. If we needed this in the past, we’d resort to trickery like fading in a pseudo-element with the new gradient colors or transitioning the background-position of a wider-than-the-element gradient to fake it.

Sounds like now we can…

@property --gradient-start {   syntax: "<color>";   initial-value: white;   inherits: false; }  .el {   --gradient-start: white;   background: linear-gradient(var(--gradient-start), black);   transition: --gradient-starty 1s; }  .el:hover {   --gradient-start: red; }

Presumably, that works now because we’ve told CSS that this custom property is a <color> so it can be treated/animated like a color in away that wasn’t possible before.

Reminds me of how when we use the attr() function to pull like data-size="22px" off an element, we can’t actually use the <length> 22px, it’s just a string. But that maybe-someday we’ll get attr(data-size px);

I have no idea when @property will actually be available, but looks like Chrome will ship first and there are positive signals from Safari and Firefox. 👍

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Helping Browsers Optimize With The CSS Contain Property

There is a growing number of things that we have to do to help the browser achieve for peak performance.

  • Responsive image syntax has several. For example, needing to tell the browser how large the image will be in our layout with the sizes attribute and how big the images are with w descriptors.
  • Regular images have always had width and height presentational attributes, which have always been somewhat useful and are now really useful.
  • The will-change property in CSS is sometimes needed for more performant animation.
  • If you want any offline capability at all, or exotic performance improvements, you have to write custom service workers code.

Now we have CSS contain, the point of which is to tell the browser about your layout so it can do things faster. You don’t need to use it, but it’s a nice enhancement for optimization.

.item {   contain: layout;   contain: paint;   contain: size;    contain: content; /* same as `layout paint` */   contain: strict;  /* same as `layout paint size` */ }

Here’s Rachel specifically about layout:

With layout containment enabled, the browser knows that nothing outside the element can affect the internal layout, and nothing from inside the element can change anything about the layout of things outside it. This means that it can make any possible optimizations for this scenario.

These things have real implications, like kicking off z-index contexts, clearing floats, and hiding overflow, so they definitely fall into the “know what you are doing” territory. Rachel is right though about design systems:

I would suggest that this is a great thing to add to any components you create in a component or pattern library, if you are working in this way it is likely each component is designed to be an independent thing that does not affect other elements on the page, making contain: content a useful addition.

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Weekly Platform News: CSS column-span Property, ADA applies to Websites, Auto-generated Image Descriptions

In this week’s roundup: multi-column layouts gain wide support, the ADA means more A11y for retailers, and Google is doing something about all the empty image alt attributes in the wild.

The CSS column-span property will soon be widely supported

The CSS column-span property, which has been supported in Chrome and Safari since 2010 (and IE since 2012), is finally coming to Firefox in version 71 (in December).

This feature enables elements that span across all columns in a multiple-column layout. In the following demo, the headings span across both columns.

article {   column-count: 2; }  h2 {   column-span: all; }

See the Pen
Demo of CSS column-span: all
by Šime Vidas (@simevidas)
on CodePen.

(via Ting-Yu Lin)

The Americans with Disabilities Act applies to websites

In the United States, the Americans with Disabilities Act (ADA) applies to websites, which means that people can sue retailers if their websites are not accessible.

Domino’s Pizza’s appeal was recently turned down by the Supreme Court, so the lawsuit against them for failing to make their website accessible to screen reader users will now resume in district court.

Guillermo Robles, who is blind, filed suit in Los Angeles three years ago and complained he had been unable to order a pizza online because the Domino’s website lacked the software that would allow him to communicate. He cited the ADA, which guarantees to people with a disability “full and equal enjoyment of the goods and services … of any place of public accommodations.”

(via David G. Savage)

Google announces automatically generated image descriptions for Chrome

When used with the VoiceOver screen reader, Chrome can now automatically generate image descriptions for images that do not have proper alt text (<img alt> attribute). Google has already created more than 10 million image descriptions, but they are not meant to replace alt text written by humans.

Image descriptions automatically generated by a computer aren’t as good as those written by a human who can include additional context, but they can be accurate and helpful.

This new accessibility feature, called “Accessibility Image Descriptions,” may not be enabled by default in your version of Chrome, but you can enable it manually on the chrome://flags page.

(via Dominic Mazzoni)

More news…

Read even more news in my weekly Sunday issue that can be delivered to you via email every Monday morning.

More News →

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Weekly news: Truncating muti-line text, calc() in custom property values, Contextual Alternates

In this week’s roundup, WebKit’s method for truncating multi-line text gets some love, a note on calculations using custom properties, and a new OpenType feature that prevents typographic logjams.

Truncating mutli-line text

The CSS -webkit-line-clamp property for truncating multi-line text is now widely supported (see my usage guide). If you use Autoprefixer, update it to the latest version (9.6.1). Previous versions would remove -webkit-box-orient: vertical, which caused this CSS feature to stop working.

Note that Autoprefixer doesn’t generate any prefixes for you in this case. You need to use the following four declarations exactly (all are required):

.line-clamp {   overflow: hidden;   display: -webkit-box;   -webkit-box-orient: vertical;   -webkit-line-clamp: 3; /* or any other integer */ }

(via Autoprefixer)

Calculations in CSS custom property values

In CSS, it is currently not possible to pre-calculate custom property values (spec). The computed value of a custom property is its specified value (with variables substituted); therefore, relative values in calc() expressions are not “absolutized” (e.g., em values are not computed to pixel values).

:root {   --large: calc(1em + 10px); }  blockquote {   font-size: var(--large); }

It may appear that the calculation in the above example is performed on the root element, specifically that the relative value 1em is computed and added to the absolute value 10px. Under default conditions (where 1em equals 16px on the root element), the computed value of --large would be 26px.

But that’s not what’s happening here. The computed value of --large is its specified value, calc(1em + 10px). This value is inherited and substituted into the value of the font-size property on the <blockquote> element.

blockquote {   /* the declaration after variable substitution */   font-size: calc(1em + 10px); }

Finally, the calculation is performed and the relative 1em value absolute-ized in the scope of the <blockquote> element — not the root element where the calc() expression is declared.

(via Tab Atkins Jr.)

Contextual Alternates

The “Contextual Alternates” OpenType feature ensures that characters don’t overlap or collide when ligatures are turned off. You can check if your font supports this feature on wakamaifondue.com and enable it (if necessary) via the CSS font-variant-ligatures: contextual declaration.

(via Jason Pamental)

Announcing daily news on webplatform.news

I have started posting daily news for web developers on webplatform.news. Visit every day!

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Get a CSS Custom Property Value with JavaScript

Here’s a neat trick from Andy Bell where he uses CSS Custom Properties to check if a particular CSS feature is supported by using JavaScript.

Basically, he’s using the ability CSS has to check for browser support on a particular property, setting a custom property that returns a value of either 0 or 1 (Boolean!) and then informing the JavaScript to execute based on that value.

Here’s his example:

.my-component {   --supports-scroll-snap: 0; }  @supports (scroll-snap-type: x mandatory) {   .my-component {     --supports-scroll-snap: 1;   } }
const myComponent = document.querySelector('.my-component'); const isSnapSupported = getCSSCustomProp('--supports-scroll-snap', myComponent, 'boolean');

As Andy mentions, another common way to do this is to use pseudo elements on the body element and then access them with JavaScript, but this approach with @supports seems a whole lot cleaner and less hacky to me, personally. I wonder how many more intuitive things we’ll find we can do with CSS Custom Properties because this is an interesting case where CSS instructs JavaScript, instead of the other way around.

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