[css-fonts-4] font-language-override tag is not necessarily 3 letters

[jfkthame]

The spec says that the value of font-language-override is a "single three-letter case-sensitive OpenType language system tag". However, while it's true that most OT langsys tags are three letters, this is not actually a requirement; the OT spec has a few 4-character tags.

(Actually, they're all 4-character, because the ones that look like 3-letter tags are padded with a trailing space.)

The characters involved are not necessarily limited to letters, either. Although the spec says that "[b]y convention, registered language system tags use three or four capital letters", the list has a couple of examples that include a digit 0.

[heycam]

For those that have a space as the fourth character, should we match if the space is included as well as if it's excluded (the current behavior)?

[jfkthame]

I think so; it would seem a bit perverse not to, given that's the "real" tag. Accepting the 3-letter form is simply a convenience.

More generally, any string shorter than 4 characters should be padded with trailing spaces to a length of 4; in theory, a font could implement a language system with a tag "X " -- although I'm sure such a tag will never end up in the registered list -- and font-language-override: "X" should work to access it.

[svgeesus]

Thanks, good catch. I will fix the spec to note that this is a 4-character string and that shorter strings specified in CSS get trailing-space padded to 4. Sound good?

[jfkthame]

SGTM, thanks.