[css-masking] Behavior of clip on position:fixed descendants is unspecified

[smfr]

CSS Masking defines the (deprecated) clip property here: https://drafts.fxtf.org/css-masking/#clip-property

It notes that the property only applies to position:absolute elements (UAs also apply it to position:fixed so this could use clarification). However, the text does not say that clip only applies to containing-block descendants, nor does clip create a stacking context.

Given that, I'd expect in this example that the clip would NOT apply to the fixedpos child:

<style>
#absolute {
    position: absolute;
    background-color: chartreuse;
    top: 100px;
    left: 100px;
    width: 200px;
    height: 200px;
    clip: rect(0, 200px, 20px, 0px);
}
#fixed {
    position: fixed;
    top: 50px;
    left: 50px;
    background-color: salmon;
    width: 100px;
    height: 100px;
}
</style>
<div id="absolute">
    <div id="fixed"></div>
</div>

and yet all UAs agree that it does. This makes it different from the overflow property, which only clips containing-block descendants, and different from clip-path that triggers a stacking context. As an implementor, I'd prefer that clip isn't so special, but at least the spec should describe its implemented behavior.

[smfr]

@emilio @bfgeek any thoughts?

[yisibl]

Hi @smfr, background-clip: text has a similar problem, can you fix it in Safari?