Follow-up #24250 (comment)

In developing the Global Styles sidebar, we ran into the issue of having to convert to/from units the font size property. The issue is that font size values are serialized as px values in the post editor, so we did the same for the site editor.

This makes it so the global styles sidebar has to absorb that processing from theme.json data, which was done via utility functions fromPx/toPx.

This is to discuss further steps on how to best manage font-size units. Can we make it so that the UI component is responsible to absorb this complexity?