[css-fonts-5] font-size-adjust with missing metrics

[fantasai]

When the metric needed for font-size-adjust cannot be derived from the font (e.g. metric isn't registered / necessary glyphs are missing), what do we do?

The units fall back to some magic number, but probably for font-size-adjust we should just not adjust the font’s size.

[fantasai]

Relatedly, for the from-font keyword, should we be picking from the first available font that is able to provide the desired metric?

[frivoal]

Indeed, no adjustment when missing glyphs / metrics is probably the correct thing to do. "Best guess" fallback values are not particularly likely to be close to what the font actually looks like, and will as easily be too big or too small. Some of the time, the adjustment would end up being in the right direction, but it would be just as likely to do the opposite of what you want.

For ic, 1em is the correct size for the vast majority of CJK fonts, so it might still be tempting to synthesize the missing measurement to that value, but then again, if your font is missing a glyph for 水, how likely is it that it actually is a CJK font?

[jfkthame]

For ic, 1em is the correct size for the vast majority of CJK fonts, so it might still be tempting to synthesize the missing measurement to that value, but then again, if your font is missing a glyph for 水, how likely is it that it actually is a CJK font?

If it's a CJK font that has been subsetted to optimize for specific content, it could well have lost its 水.

[kidayasuo]

For ic, 1em is the correct size for the vast majority of CJK fonts, so it might still be tempting to synthesize the missing measurement to that value, but then again, if your font is missing a glyph for 水, how likely is it that it actually is a CJK font?
If it's a CJK font that has been subsetted to optimize for specific content, it could well have lost its 水.

An actual case found in the market is fonts that only support Kana. They do not contain the character "水" and they are typically full-width.

[shivamidow]

Another viable approach would entail establishing fallback values for each font metric by referencing the existing fallback values utilized by browser engines, as discussed in #8792. For instance, both Blink and Webkit rely on the sum of ascent and descent for the advance height of 水 when 水 or vertical glyph information is unavailable.

[drott]

I am in favor of a behavior of "no adjustment" when the metric is not available.