:defined
Matches any element that has been officially 'defined' by the browser. This includes all standard HTML elements and any custom elements that have been registered via JavaScript.
Quick example
my-custom-element:defined {
display: block;
/* only shows the custom element once it's been correctly registered */
}:defined Browser Support
Widely available Since 2020 95% global usage
This feature is well established and works across many devices and browser versions. It has been available across browsers since 2020.
Safe to use without fallbacks.
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