To reproduce:

1. start emacs with -Q
2. Visit (C-x C-f) the attached file (foo.html).

When I do this with 26.2.90, the text starting with the apostrophe in
"doesn't", and ending at the end of file, is fontified as a string.

When I do this with 26.2-rc1, the text is fontified as I would expect
(HTML tags are in blue, everything else is in the default black or dark
gray).