It seems to me the CSS Flexible Box spec says, if the part of flex-basis is omitted in a declaration of flex shorthand, the specified value of flex-basis should be "0". And given flex-basis accepts "0" as a <width> value, I believe it means "0px" rather than "0%" here (although they are effectively the same for layout, they are different from CSSOM.)

I didn't test the code, but from the expected behavior of layout/style/test/test_flexbox_flex_shorthand.html test, I suppose the current behavior doesn't match the spec.

I recall the spec requiring 0% (with the percent) at some point. If it doesn't now, then there was likely a spec change.

I guess you're right that the spec at least required 0% at some point, because all browsers I test so far (Firefox, Chrome, Edge) all agree with that behavior.

The change is: https://github.com/w3c/csswg-drafts/commit/2c446befdf0f686217905bdd7c92409f6bd3921b

From the discussion mentioned in the commit, it seems to me changing from 0% to 0px may actually affect the layout. And given that all browsers currently agree with setting it to 0%, I don't think we should risk changing the behavior. So we should just make stylo align with other browsers.

Submitted servo/servo#17073 for making Servo align with browsers.

Landed in https://hg.mozilla.org/integration/autoland/rev/19bb59dbbfc1 https://hg.mozilla.org/integration/autoland/rev/4b4e9dbc0259 Marking as FIXED.