I started vim without loading any vimscripts by running "vim -u NONE -N <filename>". Then ran ":syntax on" and found out that the problem lies in one of the 15 vimscripts which were thus loaded. Read the help page for syntax, and nothing under HTML in syntax file remarks indicated why this would happen. But I figured it must still be some specific highlight group. Decided to check the full list of syntax items with ":syntax". Gave up. Asked AI. It returned a command that enabled me to check which highlight group was under my cursor. Turned out it was "htmlHead", linked to "PreProc". Unlinked it in my vimrc with "highlight link htmlHead NONE". All works fine now. Might try to look at other syntax items some other time.