Consider adding a workaround for the "mce_css" filter

[azaozz]

The mce_css filter is the "default" way for adding stylesheets inside the editor. Can be somewhat problematic for the Classic block as now these stylesheets will have to be added to the whole page (as the Classic block instance is inline).

[azaozz]

There are around 100k hits for mce_css in a quick GH search. That's "not representative" as most seem to be forked/copies, but seems many WP themes and plugins use that filter.

[pento]

I don't really like the idea of adding mce_css support to the block editor, I expect we'll run into the same issues as trying to load old editor style sheets.

I think we can reasonably call mce_css unsupported in the block editor.