[css-text-decor][css-ruby] Underline position for Japanese text with ruby

Hi,

Recently, Masayuki mentioned that, for Japanese, if a piece of text in
vertical writing mode has both ruby and underline, the underline should be
outside ruby annotation, like:

分わ|
か |
る |

However, implementors who are not aware of this will tend to implement that
as:

分|わ
か|
る|

(This is actually the current behavior of Firefox Nightly)

I know the current spec says the exact position of underline is UA-defined,
but I guess it at least worths a note. Actually, I don't think this is a
behavior could be covered by "UA-defined".

As an implementor, I feel it looks hard to implement, but probably not
something impossible.

Also it seems to me this is not mentioned in JLReq. I guess we should also
cc people there and ask their opinion about this.

- Xidorn