Tag: Parent

Meet `:has`, A Native CSS Parent Selector

The reasons that are often cited that make container queries difficult or impossible is things like infinite loops—e.g. changing the width of an element, invalidating a container query, which changes the width again, which makes the container query take effect, etc. But that was solved with containment. A “parent selector”, or :has as it is now been officially dubbed (I like it, that’s how jQuery rolled, although Adrian pointed out a tweet noting that it’s more versatile), has traditionally had similar problems. Things like requiring “multiple pass” rendering which is too slow to be acceptable.

Brian Kardell says:

Primarily, even without :has() it’s pretty hard to live up to performance guarantees of CSS, where everything continue to evaluate and render “live” at 60fps. If you think, mathematically, about just how much work is conceptually involved in applying hundreds or thousands of rules as the DOM changes (including as it is parsing), it’s quite a feat as is.

Engines have figured out how to optimize this based on clever patterns and observations that avoid the work that is conceptually necessary – and a lot of that is sort of based on these subject invariants that has() would appear to throw to the wind.

The fact that there is a spec now is super encouraging, and that it has Igalia’s eye on it. Apparently, some of the performance problems have either been surmounted or, through testing, determined to be negligible enough to remain a shippable feature.

Adrian Bece digs into it all!

The team at Igalia has worked on some notable web engine features like CSS grid and container queries, so there is a chance for :has selector to see the light of day, but there is still a long way to go.

What makes relational selector one of the most requested features in the past few years and how are the developers working around the missing selector? In this article, we’re going to answer those questions and check out the early spec of :has selector and see how it should improve the styling workflow once it’s released.

Let’s cross our fingers. I’ve been watching this for 10 years and trying to document use cases.

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A Use Case for a Parent Selector

Having a “parent selector” in CSS is mentioned regularly as something CSS could really use. I feel like I’ve had that thought plenty of times myself, but then when I ask my brain for a use case, I find it hard to think of one. Well, I just had one so I thought I’d document it here.

A classic parent/child:

<div class="parent">   <div class="child"></div> </div>

Say it makes a lot of sense for this parent to have hidden overflow and also for the child to use absolute positioning.

.parent {    overflow: hidden;    position: relative; }  .child {    position: absolute;  }

Now let’s say there’s one special circumstance where the child needs to be positioned outside the parent and still be visible. Hidden overflow is still a good default for the vast majority of situations, so it’s best to leave that rule in place, but in this very specific situation, we need to override that overflow.

.special-child {    position: absolute;     bottom: -20px; /* needs to be slightly outside parent */ }  /* Not real, but just to make a point */ .special-child:parent(.parent) {    overflow: visible; }

That selector above is fake but it’s saying, “Select the parent of .special-child,” which would allow that override as needed. Maybe it’s like this:

.parent < .special-child {  }

…which is selecting the element on the left rather than the right. Who knows? Probably both of those are problematic somehow and the final syntax would be something else. Or maybe we’ll never get it. I have no idea. Just documenting a real use case I had.

You might be thinking, “Why not just use another special class on the parent?” I would have, but the parent was being injected by a third-party library through an API that did not offer to add a class of my choosing on it. Ultimately, I did have to add the class to the parent by writing some custom JavaScript that queried the DOM to find the .special-child, find the parent, then add the class there.

Do y’all have some other use-cases for a parent selector?

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Oh Hey, Padding Percentage is Based on the Parent Element’s Width

I learned something about percentage-based (%) padding today that I had totally wrong in my head! I always thought that percentage padding was based on the element itself. So if an element is 1,000 pixels wide with padding-top: 50%, that padding is 500 pixels. It’s weird having top padding based on width, but that’s how it works — but only sorta. The 50% is based on the parent element’s width, not itself. That’s the part that confused me.

The only time I’ve ever messed with percentage padding is to do the aspect ratio boxes trick. The element is of fluid width, but you want to maintain an aspect ratio, hence the percentage padding trick. In that scenario, the width of the element is the width of the parent, so my incorrect mental model just never mattered.

It doesn’t take much to prove it:

See the Pen
% padding is based on parent, not itself
by Chris Coyier (@chriscoyier)
on CodePen.

Thanks to Cameron Clark for the email about this:

The difference is moot when you have a block element that expands to fill the width of its parent completely, as in every example used in this article. But when you set any width on the element other than 100%, this misunderstanding will lead to baffling results.

They pointed to this article by Aayush Arora who claims that only 1% of developers knew this.

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