Follow-up to T217722.

How to reproduce:

• Start editing a page that has "sanitized-css" content model (example of page).
• Add some unsupported CSS (for example :is(.foobar) selector).
• Do "Show preview" or "Show changes".

Expected: A warning is supposed to be shown, pointing out the unsupported CSS. (The warning from the right picture of T266157#7450639, below the preview note.)

Current: No warning is shown.

(Though, when doing "Publish changes", the publishing is prevented and a warning pointing out the unsupported CSS is shown, as expected.)

What I tried, without success:

• Setting the interface language to English, to avoid using the custom MediaWiki:Previewnote.
• Disabling the Live preview feature ("Show preview without reloading the page").