The bug is also present in Emacs 26.

It looks like `mhtml--extend-font-lock-region' and
`font-lock-extend-region-multiline' are squabbling about where to
leave `font-lock-beg'.

In this particular example, `mhtml--extend-font-lock-region' wants to
leave `font-lock-beg' at 40 while `font-lock-extend-region-multiline'
wants it at 39, resulting in an endless back and forth.

CC-ing Tom in case he has some insight into how it should be fixed.

-- Simen