(The original topic from my first email in this thread is probably not relevant, as Japanese people have confirmed that the current behavior of Firefox makes sense, and there is probably no clarification needs to be added to the spec. But there is another related issue raised in the thread.)

Copy from my old email:

Masayuki-san mentioned another issue in Twitter: https://twitter.com/d_toybox/status/615061112019513344

This does seem to be an issue.

In our current spec, the positions of underline and ruby annotation are:

• if underline position is right, the underline is put to the right of the text content [1]
• the margin box of rbc and rtc are put without intervening space [2]

This means, if both underline and ruby annotation are specified, and author doesn't add any space to either the base or the annotation, the underline and annotation may overlap each other in some fonts.

I have no idea how this issue could be solved. If we want to change the position of underline, the only option is to move it inwards, which may make it overlap with the base text. Hence we cannot do that.

Changing position of ruby annotation isn't quite a good option either, since it could break interop. But given the interop has already been broken due to the height of inline box being UA-defined [3], we probably can do something there?

[1] http://dev.w3.org/csswg/css-text-decor-3/#underline-right
[2] http://dev.w3.org/csswg/css-ruby/#ruby-layout
[3] https://lists.w3.org/Archives/Public/www-style/2015May/0276.html