To shed some light on what's going on. As you probably know, HTML originates from SGML. In SGML they could write something like that:
<!ELEMENT some serious stuff
Some stupid comments
that can even spread across multiple lines
more serious stuff
-- More stupid comments --
BTW. Note there's no space between <! and ELEMENT, but ...
This was a bug in Vim's HTML syntax, which @Matt has fixed with lightning-speed.
Upgrade to an upcoming version of Vim to fix this, or simply copy runtime/syntax/html.vim from Matt's fork into ~/.vim/syntax to try out his fix immediately.