I have the following in my .vimrc to highlight lines longer than 80 chars:
highlight OverLength ctermbg=red ctermfg=white guibg=#592929 match OverLength /\\%
Sounds like you might want something like:
autocmd FileType html,xml highlight OverLength ctermbg=red ctermfg=white guibg=#592929 autocmd FileType html,xml match OverLength /\%81v.*/
Seems to work for me anyway :-)