bug#30263: 26.0.91; whitespace-mode in mhtml-mode

[Charles A. Roelli]

src/emacs -q
C-x C-f test/manual/indent/html-multi-2.html RET
M-x whitespace-mode RET

It looks like whitespace is not shown within <style> (CSS) and <script> (JS)
tags the same way as it is in HTML parts of the buffer.  For example, line 14
correctly shows as:

        <style type="text/css">

with a lightly-colored "»" in column 1, and a lightly-colored leading
whitespace, whereas line 15 shows as:

»       h1 {

with no coloring, even though the leading whitespace is the same (one TAB
character).