Bugzilla

The spec language at:
https://drafts.fxtf.org/css-masking-1/#the-mask-image
> A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity [CSS3COLOR] does for values other than 1.

seems clear that all these properties should be handled the same as `opacity`, so they should be applied also for the block-in-inline case.

Which leaves my original question unanswered: how are these fragments supposed to be joined together?

I think current layout is reasonable. Basically, treat the inline fragments as if the blocks don't exist, and the blocks (each block element separately though) as if the inlines don't exist.  I'll file a spec issue though, because the spec needs to define this more precisely...

The spec language at:
https://drafts.fxtf.org/css-masking-1/#the-mask-image
> A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity [CSS3COLOR] does for values other than 1.

seems clear that all these properties should be handled the same as `opacity`, so they should be applied also for the block-in-inline case.

Which leaves my original question unanswered: how are these fragments supposed to be joined together?

I think our current layout is reasonable. Basically, treat the inline fragments as if the blocks don't exist, and the blocks (each block element separately though) as if the inlines don't exist.  I'll file a spec issue though, because the spec needs to define this more precisely...