X-Debbugs-CC: Stefan Monnier <monnier@iro.umontreal.ca>
Tags: patch

In an nxml-mode buffer, put the following text:

<x a="foo" b='bar'/>

Only "foo" is fontified in as a string, while bar is not.  This is a
regression since Emacs 25.3.  The patch below seems solves problem,
though I'm not entirely sure about it: there are some confusing comments
in sgml-mode.el about drawbacks and tradeoffs of recognizing single
quotes that I'm not really following (are they applicable to SGML only,
and not XML?).

There's also the difference that attribute values are now fontified with
font-lock-string-face rather than nxml-attribute-value, though I'm not
sure that's worth fixing.