feature request: @font-face coverage

[dseomn]

I'm thinking of adding Noto fonts to my site, but I sometimes use characters from all over Unicode and the Noto fonts are split up by script (mostly). So each time I use a new Unicode script in the html text, I'd need to add the appropriate font. Would it make sense to add a test here that for any html text covered by a css font-family that references one or more custom fonts, every character¹ in that text must be covered by at least one of the custom fonts?

So if I have font-family: "Noto Serif", serif and <p>Ogham text is often surrounded by ᚛ and ᚜, then html-proofer could raise an error that ᚛ and ᚜ are not in "Noto Serif", even if they are in the fallback serif. The error could be fixed by removing the font-family entirely, changing it to font-family: serif, or changing it to font-family: "Noto Serif", "Noto Sans Ogham", serif

Footnotes

1. It's probably a bit more complicated than every character. E.g., 🏴󠁧󠁢󠁳󠁣󠁴󠁿 is a black flag emoji followed by tag characters. So the combined Scottish flag should be in the custom fonts, but it doesn't matter if the individual tag characters are in the custom fonts. If you think the idea is good, then I think extended grapheme clusters might be the right unit to look up in a font. ↩

[gjtorikian]

I think the idea is interesting, but it sounds complicated to implement. I imagine the proofer would have to download the fonts applied, then determine which glyphs/characters are not found in that font—meaning each and every character in the text would need to be looked up?

[dseomn]

Yes, I think that is what would be needed. Fonts and rendering are somewhat outside my areas of experience, so take this with a grain of salt, but I was thinking there might be some way to get a rendering library to do some or most of that instead of implementing all the details here? E.g., Firefox's all fonts on page UI presumably does most of this work, but I don't know if the rendering engine's API is easy to use to get the right info here. Or maybe something like Pango provides an easier API to render a <span> of text and get a list of fonts that it used?