Question

As stated in, When did single quotes in HTML become so popular? and Jquery embedded quote in attribute, the Wikipedia entry on HTML says the following:

The single-quote character (‘), when used to quote an attribute value, must also be escaped as ' or ' (should NOT be escaped as ' except in XHTML documents) when it appears within the attribute value itself.

Why shouldn’t ' be used? Also, is " safe to be used instead of "?

Answer 1

" 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.