Highlight API: Need to clarify behavior for replaced elements, tables, etc.

[BoCupp-Microsoft]

This API is intended only to create text highlights. The explainer should probably clarify the behavior when a range covers replaced elements like images or controls and when multiple table cells are selected.

Specifically I'm referring to the rendering of the highlight and whether we intend to create a "frosted" appearance as images are covered by a highlighted range, etc.

[sanketj]

The existing highlight pseudo element spec (https://www.w3.org/TR/css-pseudo-4/#highlight-bounds) talks about a "highlight overlay", one for each type of highlight pseudo, that is rendered on top of "text and replaced content". Since ::highlight is just another type of highlight pseudo element, it will follow the same model. So, for example, if the user is implementing a custom selection with the highlight API, their selection will create a "frosted" appearance over images in the same way that native selection does.