you sure can if we resolve to make a text editor instead of a web browser!
Hmm, that might not be a bad idea! We could ban CSS altogether: just leave some markup tags, maybe whatever you can do in Markdown + tables. No colors, no small print, no images. (We could even use something like gemtext as the format instead of HTML, but that wouldn’t be backward compatible with clients.)
But I don’t see any email clients with somewhat significant market share going through with this :(
(While on it, can we also ban loading images from third-party servers?)