UAX14 covers a lot of things, much of which is tailorable, some of which is required by css-text, and some of which is overriden by css-text. Could you be a bit more specific about which part of UAX14 you're thinking of adopting?

For what it's worth, I think that what @jfkthame is saying in https://bugzilla.mozilla.org/show_bug.cgi?id=1848049 is very reasonable, and as far as I can tell, is neither required nor forbidden by css-text.

The spec deliberately leaves line-breaking fairly open-ended, and doesn't require UAX14 compatibility, because UAX14 is known to not be an ideal line-breaking implementation, merely a reasonable baseline to start from. So I'm not sure the spec should change here: as Florian says, it allows what Mozilla wants to do already.

@makotokato Is it OK to close this issue, or do you want us to raise it with the CSSWG?

I discussed this with @makotokato and based on @jfkthame's feedback I think we'd like to add an informative note regarding the sort of deviations from UAX#14 that implementers might want to consider for compatibility's sake. I can try to draft that next week if I'm available (feel free to steal it from me though) but for now I'd like to keep this issue open.

I made up a strawman note over in #9997 (nearly 10,000!). I'll be on leave for the next few weeks so I can't promise I'll be able to address any feedback. Feel free to take it over and fix as you like!