[css-fonts-3][css-fonts-4] letter-spacing disables optional ligatures. #2644

[bungeman]

A non-default value for letter-spacing should disable all
optional ligatures, not just common ligatures. Defer to css-text for
optional ligature definition.

Test that letter-spacing disables discretionary ligatures in particular at
https://chromium-review.googlesource.com/c/chromium/src/+/1294874

[svgeesus]

Thanks for the PR and test.

Defer to css-text for optional ligature definition.

Do you mean this wording:

When the effective spacing between two characters is not zero (due to either justification or a non-zero value of letter-spacing), user agents should not apply optional ligatures, i.e. those that are not defined as required for fundamentally correct glyph shaping

or this non-normative note?

Note: In OpenType, required ligatures are expected to be associated to the rlig feature. All other ligatures are therefore considered optional. In some cases, however, UA or platform heuristics apply additional ligatures in order to handle broken fonts; this specification does not define or override such exceptional handling.

I agree that optional, rather than just common, ligatures is correct here.

Oh - the ipr bot is complaining, could you link your GitHub account with your W3C account please?

[bungeman]

Since the checks are now passing, I assume that I've finally hit all the buttons and done all of the verifying required.

By "Defer to css-text for optional ligature definition." I mean both of the sections you mention. css-text here is defining optional ligatures as "those that are not required for fundamentally correct glyph shaping" and that is a great general definition, especially in the css-text specification. The note about OpenType is also good in that it clarifies what this means for the 95% use case. If there were also a note about TrueType shaping in there it would hit 99.9% or so. However, this note is already straying a bit into the css-fonts specification area, so I understand why it is a note.