[css-fonts] Computed value of font-feature-settings

The current text of CSS Fonts 4 seems to contradict itself here.

On the one hand, 6.12 says

Computed value: as specified

Suppose the specified value is

font-feature-settings: "abcd", "spqr", "abcd" 2;

Note that the "abcd" feature has been specified twice, first with the implied value of 1, and later with an explicit value of 2.

So what is the computed value of font-feature-settings? If it's "as specified", then it should be exactly this list of three features, including both entries for "abcd".

However, the text goes on to say, a few paragraphs later,

The computed value of font-feature-settings is a map, so any duplicates in the specified value must not be preserved. If the same axis name appears more than once, the value associated with the last appearance supersedes any previous value for that axis.

Currently, of the browsers I've tried, only Safari appears to do this, returning "spqr", "abcd" 2 from getComputedStyle; both Firefox and Chrome return the list "as specified" including the first (superseded) occurrence of "abcd".

We should confirm which behavior is wanted ("as specified" or "with duplicates removed"), and adjust the spec accordingly. (My inclination is to require the duplicate-removal behavior, as implemented in Safari, so that would imply Firefox and Chrome should fix their bugs here.)

(Note that the similarly-structured font-variation-settings property defines its computed value differently:

Computed value: the keyword 'normal' or a list, each item a string paired with a number

It then has similar text confirming that duplicates should be removed:

If the same axis name appears more than once, the value associated with the last appearance supersedes any previous value for that axis. This deduplication is observable by accessing the computed value of this property.

So maybe a similar computed value description should be used for font-feature-settings, instead of "as specified".)

Read CSS Fonts 4 §6.12 and the linked font-variation-settings definition, then compare their computed-value wording with the duplicate-handling prose. Check the reported getComputedStyle behavior in Safari, Firefox, and Chrome. Done means the Working Group has resolved the intended behavior and the specification text is consistent with that decision.