Add feature tag(s) for legible/disambiguated forms

[kitgrose]

Description

Sans serif typefaces can have many glyphs with ambiguous forms, such as uppercase O and zero, or lowercase l, uppercase I and 1. While that's normally of only minor impact, this can make automatically-generated text like usernames, passwords, and serial numbers difficult to read.

To mitigate this impact, many typefaces provide a stylistic set with alternates for character disambiguation (e.g. Inter uses ss02 for this purpose, and Apple's San Francisco font uses ss06), and there's already a zero tag for slashed zero.

That's manageable for traditional print design, but isn't ideal for the web where the exact font file that is loaded can be affected by factors outside the author's control. If an author sets their font family to something like ui-sans-serif (which dynamically maps to the system's default UI sans serif font), and then enables ss06 for elements on the page which would benefit from disambiguated forms, it may appear to do the right thing on their Mac, but in doing so introduce totally unexpected side-effects for users on different platforms, for those who override the font family in their browser preferences, or for fallback fonts if a font doesn't load properly (if the user loaded Inter with ss06 enabled, every character would be outlined in a box!).

I propose something like lgbl (meaning legible) as a new feature tag that could be mapped to the most legible/differentiated glyphs in new releases of existing fonts to define its purpose semantically. Failing that, more specific feature tags for things like l with finial, or serif I, or 1 with flag would allow us to combine features to mitigate more common issues (but I think that's probably less ideal, since some fonts may have more or fewer ambiguous glyphs).

This would provide an accessibility benefit in digital applications and in many cases would require no new glyphs be created.

Page URL

https://learn.microsoft.com/en-us/typography/opentype/spec/ttoreg

Content source URL

https://github.com/MicrosoftDocs/typography/blob/live/typographydocs/opentype/spec/ttoreg.md