:defined can be used to target not yet defined custom elements
Written by Stefan Judis
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This post is part of my Today I learned series in which I share all my web development learnings.
Today I learned that there is a :defined pseudo class in CSS. With a little bit of CSS magic using :not(:defined) one could style custom elements that are currently loaded. I love that!
<!DOCTYPE html>
<html lang="en">
<head>
<style>
/* style custom elements that are not defined yet */
:not(:defined) {
border: 2px solid red;
}
</style>
</head>
<body>
<some-elem>Hello world!</some-elem>
<script>
// assume that this code is loaded async
class SomeElem extends HTMLElement {}
customElements.define('some-elem', SomeElem);
</script>
</body>
</html>
If you want to check the spec definition of :defined is rather light though. 😆
And if you want to see the selector in action I tweeted a short video showing it.
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Frontend nerd with over ten years of experience, freelance dev, "Today I Learned" blogger, conference speaker, and Open Source maintainer.