[css-text-decor-4] Give some guidance on auto thickness of underlines #1935
Description
See also Brad's comments from #239
The em note is a good tip for getting widths relative to font-size, but I wouldn't want "strongly encouraged". 'ch' and 'ex' would also be good choices there. Some authors want type-relative thicknesses, others (like me) think that looks weird, and would never want more than 2px or less than 1px. I also wouldn't want fractions of a device pixel (which might be rendered as less than full opacity).
The L4 text should maybe say that the 'auto' value is generally expected to be the same thickness as an underscore character, or something, and might also be derived from PostScript/OpenType tables. UAs can also set lengths in their UA style sheet, if they want something different from that, such as 1px or calc(1em / 16).