[css-text-decor] Rounding of text-decoration-thickness

[emilio]

It seems Chromium does some rounding to text decoration thickness. It's not clear to me if that rounds to CSS or device pixels, I assume the later.

The spec does say to round to the nearest device pixel: https://drafts.csswg.org/css-text-decor-4/#valdef-text-decoration-thickness-length

Someone sent a patch to do something similar in Firefox, but that raises the following questions:

• At a glance, this seems similar to borders, why doesn't it use the same algorithm?

• Should this rounding be done at computed-value time rather than used-value time like borders do? I don't feel super strongly since these don't affect layout, but worth raising.

[schenney-chromium]

Looking at the code Chrome behavior seems to be inconsistent. It uses round() for a Length thickness but does not round a thickness taken from the font, or auto. That rounding would be in device pixels, I think, because it is post zoom/device pixel ratio. Chrome also sets a min width of 1px, apparently.

All this is at used-value time. It maybe hard to do it at computed-value time due to the font interactions and need for zoom and DPI. Currently I lean toward no change.