Why shouldn't `'` be used to escape single quotes?
"
is on the official list of valid HTML 4 entities, but '
is not.
From C.16. The Named Character Reference ':
The named character reference
'
(the apostrophe, U+0027) was introduced in XML 1.0 but does not appear in HTML. Authors should therefore use'
instead of'
to work as expected in HTML 4 user agents.
'
is not part of the HTML 4 standard.
"
is, though, so is fine to use.
"
is valid in both HTML5 and HTML4.
'
is valid in HTML5, but not HTML4. However, most browsers support '
for HTML4 anyway.