Emphasis marks are currently broken with sideways-lr writing mode in my proposing patchset in bug 1040668, and I'm not going to finish this support there, see bug 1040668 commnet 85. Although there is no known usecase, we may still want to support it eventually as it is a valid combination.
https://drafts.csswg.org/css-text-decor/#text-emphasis-position-property has data for Mongolian, which suggests this may be an issue, although I can't find any other references to emphasis marks in Mongolian.
Do you know how important Mongolian script is as a use case for text-emphasis?
Not sure where I got that, it's been a very long time since I did that research. However, sideways-lr isn't for Mongolian anyway, it's for horizontal scripts like Latin. It's vertical-lr that you need to work correctly for Mongolian.
AFAICS, vertical-lr works correctly with text-emphasis. Then it seems this bug doesn't really matter.
Created attachment 8973607 [details] [diff] [review] patch for adding tests (I'm cleaning my local patch queue, and this is a patch which seems to be useful for future work on this bug, so I upload it here. I don't currently have any plan to work on this bug, though.)
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