N/A value or synthesized value for TextMetrics when fonts don't have values

[kojiishi]

This is to discuss in parallel with w3c/csswg-drafts#5380 as per @annevk's advice.

Authors want to get more information from fonts, but often not all fonts can provide all information. For rendering, we might need to do some fallback, but for TextMetrics, we have options to a) provide synthesized value or b) provide the unavailability of the value.

TextMetrics is used in JavaScript, where authors can write their favorite fallback logic. I think authors would like to distinguish values in fonts (and thus should be high-quality unless the font has problems) from synthesized low-quality values.

In this specific discussion, the proposed fallback algorithm produced poor results for Blink (see picture in w3c/csswg-drafts#5380), but this may also apply to other cases, such as ideographicBaseline or hangingbaseline.

@yiyix @jfkthame