Here's an update on the overall status of the font modes work:

• Current status: Font mode mixins were added to Codex in v2.0.0-rc.1 and are currently in MediaWiki core, but the mixins aren’t being used in most skins and there are some line-height workarounds in place that need to be removed.
• Line height workarounds: See T391017. These workarounds should be removed once fixes are in place.
• Skins:
  • Vector 22:
    • We need to use the font mode mixins for each text size setting. See T393739.
    • We could either continue to apply text size settings to the html element and then exclude the “chrome” of the site (see PoC 1) or only apply text size settings to the content, where we actually want things to scale (see PoC 2).
    • This will need to be carefully tested and there may be other updates in the skin needed to make it work properly across the UI.
  • MinervaNeue:
    • We need to use the font mode mixins for each text size setting. See T396513.
    • The code that sets up the text size settings on mobile could be improved. Nice-to-have improvements are noted in the task.
    • This will need to be carefully tested and there may be other updates in the skin needed to make it work properly across the UI.
  • MonoBook: the small mixin is used to ensure that Codex components appear at a 14px base font size (close to MonoBook’s 12.7px). No more action needed. See T392253.
  • Vector legacy: needs a similar solution to MonoBook. Currently, Codex components appear at 16px. See T394685.