Tag: Backgrounds

On fixed elements and backgrounds

After just playing with apsect-ratio and being pleasantly surprised at how intuitive it is, here’s an example of CSS acting unintuitively:

If you have a fixed element on your page, which means it doesn’t move when you scroll, you might realise that it no longer acts fixed if you apply a CSS filter on its nearest ancestor. Go ahead, try it on the CodePen.

This is because applying a filter on the fixed element’s immediate parent makes it becoming the containing block instead of the viewport.

Hui Jing has more to teach in there about scrolling, rendering performance, and trickery with using pseudo elements to avoid issues.

I find this kind of thing among the most challenging CSS concepts to wrap my mind around, like Block Formatting Contexts (BFCs). A BFC Is A Mini Layout In Your Layout. 🤯

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Link Underlines That Animate Into Block Backgrounds

It’s a cool little effect. The default link style has an underline (which is a good idea) and then on :hover you see the underline essentially thicken up turning into almost what it would have looked liked if you used a background-color on the link instead.

Here’s an example of the effect on the Superfriendly site:

A journey:

  • The Superfriendly site does it with box-shadow. Turning box-shadow: inset 0 -0.07em 0 #0078d6; into box-shadow: inset 0 -0.85em 0 #a2d5fe; with a transition. Andres Cuervo ported that idea to a Pen. (I forked it to fix the “start offset” idea that was broken-seeming to me on the original).
  • You might be tempted to draw the line with a pseudo-element that’s, say, absolutely positioned within the relatively positioned link. Then you animate its height or scaleY or something. Here’s that kind of idea. Your enemy here is going to be links that break onto new lines, which box-shadow seems to handle more elegantly.
  • Another idea would be using linear-gradient with hard color stops to kinda “fake” the drawing of a line that’s positioned to look like an underline. Then the gradient can be animated to cover the element on hover, probably by moving its background-position. Here’s that kind of idea and another example we wrote up a little while back. This handles line breaks nicer than the previous method.
  • The default text-decoration: underline; has a distinct advantage these days: text-decoration-skip-ink! It has become the default behavior for links to have the underlines deftly skip around the decenders in text, making for much nicer looking underlines than any of these techniques (also: borders) can pull off. There are properties that are somewhat new that you may not be aware of that give you more control over the underline that we have traditionally had, like text-decoration-color. But there is more, like thickness and offset, that make this effect possible! Miriam Suzanne has a demo of exactly this, which only works in Firefox Nightly at the moment, but should be making the rounds soon enough.

Summary: If you need to do this effect right now in the most browsers you can, the box-shadow technique is probably best. If it’s just an enhancement that can wait a bit, using text-decoration-thickness / text-decoration-offset / text-decoration-color with a transition is a better option for aesthetics, control, and being able to understand the code at first glance.

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Managing Multiple Backgrounds with Custom Properties

One cool thing about CSS custom properties is that they can be a part of a value. Let’s say you’re using multiple backgrounds to pull off a a design. Each background will have its own color, image, repeat, position, etc. It can be verbose!

You have four images:

body {      background-position:     top 10px left 10px,     top 10px right 10px,     bottom 10px right 10px,     bottom 10px left 10px;      background-repeat: no-repeat;      background-image:     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-left.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-right.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-right.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-left.svg);    }

You want to add a fifth in a media query:

@media (min-width: 1500px) {   body {     /* REPEAT all existing backgrounds, then add a fifth. */   } }

That’s going to be super verbose! You’ll have to repeat each of those four images again, then add the fifth. Lots of duplication there.

One possibility is to create a variable for the base set, then add the fifth much more cleanly:

body {   --baseBackgrounds:      url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-left.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-right.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-right.svg),     url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-left.svg);    background-position:     top 10px left 10px,     top 10px right 10px,     bottom 10px right 10px,     bottom 10px left 10px;      background-repeat: no-repeat;      background-image: var(--baseBackgrounds); } @media (min-width: 1500px) {   body {     background-image:        var(--baseBackgrounds),       url(added-fifth-background.svg);   } }

But, it’s really up to you. It might make more sense and be easier manage if you made each background image into a variable, and then pieced them together as needed.

body {   --bg1: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-left.svg);   --bg2: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-right.svg);   --bg3: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-right.svg);   --bg4: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-left.svg);   --bg5: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-left.svg);      background-image: var(--bg1), var(--bg2), var(--bg3), var(--bg4); } @media (min-width: 1500px) {   body {     background-image: var(--bg1), var(--bg2), var(--bg3), var(--bg4), var(--bg5);   } }

Here’s a basic version of that, including a supports query:

See the Pen
Multiple BGs with Custom Properties
by Chris Coyier (@chriscoyier)
on CodePen.

Dynamically changing just the part of a value is a huge strength of CSS custom properties!

Note, too, that with backgrounds, it might be best to include the entire shorthand as the variable. That way, it’s much easier to piece everything together at once, rather than needing something like…

--bg_1_url: url(); --bg_1_size: 100px; --bg_1_repeat: no-repeat; /* etc. */

It’s easier to put all of the properties into shorthand and use as needed:

body {     --bg_1: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-left.svg) top 10px left 10px / 86px no-repeat;   --bg_2: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-top-right.svg) top 10px right 10px / 86px no-repeat;   --bg_3: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-right.svg) bottom 10px right 10px / 86px no-repeat;   --bg_4: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/angles-bottom-left.svg) bottom 10px left 10px  / 86px no-repeat;        background:     var(--bg_1), var(--bg_2),var(--bg_3),var(--bg_4); }

Like this.

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