Feature detect Style Queries Support in CSS

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# The Code

If you are here for just the code, here it is:

html {
  --sentinel: 1;
}

@container style(--sentinel: 1) {
  body {
    --supported: ; /* Space Toggle */
  }
}

With this you get a Space Toggle for you to use in your code.

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# How it works

The code works by actively trying to use a Style Query. It sets a --sentinel property on the root element and then lets the body element respond to it – using a Style Query – trying to declare the --supported custom property.

When Style Queries are supported, the result will be a --supported custom property that is set to an empty value. In browsers with no Style Queries support, the --supported property will be the guaranteed-invalid value (of initial). Yes, a Space Toggle.

With that --supported Space Toggle in place, you can then use it in your CSS:

body {
  --bg-if-support: var(--supported) green;
  --bg-if-no-support: var(--supported, red);
  background: var(--bg-if-support, var(--bg-if-no-support));
}

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# Demo

The demo below uses the code shown earlier:

See the Pen
Feature Detect Style Queries (1/2) by Bramus (@bramus)
on CodePen.

If you’re using Safari 18, you might notice it doesn’t work as expected …

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# The problem with Safari 18

While Safari 18 does come with support for (Custom Property) Style Queries, you might have noticed the previous demo does not work in it.

The culprit: A bug in which the root element cannot be a container in Safari 18 – https://bugs.webkit.org/show_bug.cgi?id=271040.

To work around this bug, you need to move everything down one level in your DOM tree. Like so:

body {
  --sentinel: 1;
}

@container style(--sentinel: 1) {
  body > * {
    --supported: ; /* Space Toggle */
  }
}

This means you can’t use --supported to conditionally style the body element itself, which might be OK for your use-case.

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# Safari 18-friendly demo

Here’s a demo of the code that also works in Safari. Note that it can’t be used to style the body element itself.

See the Pen
Feature Detect Style Queries (2/2) by Bramus (@bramus)
on CodePen.

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# Spread the word

Feel free to repost one of the posts from social media to give them more reach, or link to this post from your own blog.

📝 Feature detect Style Queries Support in CSS.

Awaiting browser support for `at-rule()`, here’s how you do it.https://t.co/uXikYbBM2S pic.twitter.com/iuyVwlAgyX

— Bram.us (by @bramus) (@bramusblog) October 6, 2024

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