A full HTML parser could be a lot slower, so I'd prefer to hold off on that for now if possible or benchmark first. Perhaps a hacky solution for now would be to strip the > character from the alt tag on paste? Before 2.1.54, the entire alt tag was stripped, so this is a recent regression.

https://forums.ankiweb.net/t/check-media-bug-symbol-in-alt-attribute/25997/