Is there a reason why .html is no longer supported (https://getkirby.com/docs/reference/system/options/content) as content file extension in v3? I worked with it in v2 without problems and because of syntax highlighting in the files (we are currently using html in 90% of the content) would like to use it in v3 too.
Out of the box, that doesn’t seem to be possible (actually, I never knew you could in Kirby 2). But if I understand the
Data class correctly, you could extend it with a custom handler.