: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.
Syntax
/* Usage 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.
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