[css-selectors-4] New selector request [:ifchecked(input-id)]

[Somnium7]

I am requesting a new selector in CSS4:
selector:ifchecked(input-id)
Which will match all elements by selector only if checkbox or radio button with id input-id is checked.

Motivation is following:
Consider code
<label for="abc">Test</label><input type="checkbox" id="abc">
where I need to emphasize label with bold when checkbox is selected. With CSS3 it is not possible. It will possible with CSS4 selector :has(#abc:checked) label[for="abc"]. However, :has selector will have perfomance problems (and because of that, possibly, will be never implemented). Requested selector will not have perfomance problems, because it behaves like normal selector, but with additional check whether corresponding checkbox/radio button is checked (which have average complexity O(1)).

[cvrebert]

One alternative to this would be #397.

[Somnium7]

This is more general and more useful.

For example, if you have a tree structure of checkboxes, with this selector you style tree parent depending on children check state.