[css-text-decor] Define how text-decoration-skip{,-ink} interacts with text-decoration #2817

text-decoration style properties are non-inherited (as they should), while text-decoration-skip and text-decoration-skip-ink are inherited (as they should as well).

They are all fine in themselves, but there would be a problem on how they should interact, or more specifically, should the "skip" behavior of a text decoration line come from the element where the text is in, or from the element where the decoration line is originated from?

For example, something like

<style>
div {
    text-decoration: underline;
    text-decoration-skip-ink: auto;
}
span {
    text-decoration-skip-ink: none;
}
</style>
<div>For <span>example</span></div>

should "example" have ink skipping?

Note that, current implementations of text-decoration-skip-ink (WebKit and Blink) don't support non-inherited text decoration, and thus they may not have this confusion at the moment. But since we want non-inherited text decoration, so it's worth making clear in the spec.