Are single quotes valid in HTML/XHTML?

Answer 1

Yes, single quotes are valid.

From the XML spec:

[10]   AttValue  ::= '"' ([^<&"] | Reference)* '"'
                  |  "'" ([^<&'] | Reference)* "'"

Answer 2

According to the spec, both are valid :-)

[9]    EntityValue    ::=    '"' ([^%&"] | PEReference | Reference)* '"'  
   |  "'" ([^%&'] | PEReference | Reference)* "'" 
[10]    AttValue    ::=    '"' ([^<&"] | Reference)* '"'  
   |  "'" ([^<&'] | Reference)* "'" 
[11]    SystemLiteral    ::=    ('"' [^"]* '"') | ("'" [^']* "'")  
[12]    PubidLiteral    ::=    '"' PubidChar* '"' | "'" (PubidChar - "'")* "'"

And the W3C-Validator accepts it as well.

Answer 3

A source of confusion on this matter seems to come from this article,

http://www.javascriptkit.com/howto/xhtml_intro.shtml

which seems to claim that single quotes are not valid in xhtml. However, since no specification is cited, and since the article also claims that lowercase names for attributes and elements are forbidden, the author may merely be expressing an editorial prohibition against using single quotes.