The following bug is a regression since 27.1 introduced on master by
commit eae028b9e2c23f84dbd130d49da65f9db1db932f (found by bisection).

To reproduce, open the attached file 'mhtml-test.html' from
'emacs -Q'. The region from position 516 to 558 (approximately the
"button {cursor: pointer; }" part) is not recognized as being part of
the style block, and doesn't receive the correct mode (HTML+CSS).