[css-text-decor] Conflicting descriptions of `text-underline-position: left` or `right` in horizontal typographic mode

[jfkthame]

I'm unclear exactly how to handle the text-underline-position property in horizontal typographic mode, when only one of the values left or right is specified.

Given for example text-underline-position: left and horizontal English text, where should the underline appear -- close to the alphabetic baseline, or at the bottom of the em box?

There seem to be two relevant statements in the spec, but as I read it, they conflict with each other.

First, we read (just below Example 4):

If left or right is specified alone, auto is also implied.

This seems to mean that the underline would typically appear close below the alphabetic baseline of the horizontal text, just as if the property were auto. This also seems consistent with the descriptions of the left and right values, which refer only to an effect in vertical typographic modes.

However, at the end of the section there is a illustration showing the effect of left and right (in vertical mode), and the caption of this illustration says:

In vertical typographic modes, the text-underline-position values left and right allow placing the underline on either side of the text. (In horizontal typographic modes, both values are treated as under.)

(my emphasis)

This seems to be saying that either left or right, alone, would have the same effect as under in horizontal mode, and so the underline would appear at the bottom of the em box.

Which should it be?

AFAICT, current implementations behave according to the first description above. If this is correct, I suggest the caption of that illustration should be changed to read:

(In horizontal typographic modes, both values are treated as auto.)

[fantasai]

Looks like I forgot to update the caption in
36fa944
See minutes of this discussion at https://lists.w3.org/Archives/Public/www-style/2017Feb/0049.html

Basically the logic was this: CJK languages need a text-underline-position value set in the default UA style sheet. Also, for compat reasons, the initial value has to be auto in horizontal writing modes. Given all that, to be more consistent with the initial value, an omitted one is auto rather than under.

Lmk if this makes sense to you or if you think we should do something different here. :)

[jfkthame]

Sounds OK to me, thanks.

[xfq]

This issue seems to be fixed in 2231d71

Should we close it?