Checking the fonts in HelveticaNeue.ttc on macOS Sonoma, both HelveticaNeue-Medium and -MediumItalic have the same weight (500) in their OS/2 table, and return the same value (0.23) for their kCTFontWeightTrait. Back in bug 931426 when the weight override pref was added, we were getting different weights from appKit for these two "Medium" faces, but that no longer appears to be an issue.
This patch makes our CoreTextWeightToCSSWeight mapping a bit more sophisticated, with the added data point of CSS/OpenType weight 500 = CoreText weight 0.23, which we're seeing here in HelveticaNeue. With this refinement in the mapping, we can drop the old override for the HelveticaNeue-MediumItalic face.
Differential Revision: https://phabricator.services.mozilla.com/D197665
4170df881e