[css-break-4][css-text-decor-4] Allow breaks to only affect text decorations

With the introduction of text-decoration-trim, we also defined that it takes box-decoration-break into account. So box-decoration-break affects whether the text decoration is split or cloned across fragments.

Though box-decoration-break also affects other parts of the box, which may be styled differently regarding fragmentation.

Therefore, I suggest a way to differentiate between the layout-related and content-related parts. I see three solutions to this:

1. Extend box-decoration-break
   Change box-decoration-break so that it allows to distinguish to what it applies, e.g. via new keywords that target layout, background, and contents.

2. Make box-decoration-break a shorthand
   Turn box-decoration-break into a shorthand for different longhands for layout, background, and contents. I.e. you'd have box-decoration-break-layout, box-decoration-break-background, and box-decoration-break-content, which take slice and clone.

3. Introduce a new property
   As suggested in https://github.com/w3c/csswg-drafts/issues/8403#issuecomment-3396895682, we might also introduce a completely separate property like text-decoration-break.

See also #8535, which refers to allow targetting only the background (and suggests to go either with option 1 or 2).

Option 2 currently seems to make the most sense to me, as it aligns well with what we already have and is backwards compatible.

Sebastian

Start by reading the proposal and the linked discussions in #8403 and #8535. Compare the three approaches for separating layout, background, and content fragmentation behavior, then follow the CSSWG discussion for a decision. Done means the Working Group has selected an approach and the relevant specification text reflects it.