(Simple) What characters are allowed in an HTML attribute name?

Question

In HTML attribute name=value pairs, what are the characters allowed for the 'name' portion? ..... Looking at some common attributes it appears that only letters (a-z and A-Z) are used, but what other chars could be allowed as well?... maybe digits (0-9), hyphens (-), and periods (.) ... is there any spec for this?

Answer 1

The values allowed are listed at http://www.w3.org/TR/html4/index/attributes.html - if you add a custom attribute, then you aren't writing HTML any more.

Answer 2

Assuming you're talking about XHTML, the XML rules apply.

See http://www.w3.org/TR/2008/REC-xml-20081126/#NT-Name

Names and Tokens

[4]     NameStartChar    ::=    ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a]    NameChar    ::=    NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]     Name    ::=    NameStartChar (NameChar)*
[6]     Names    ::=    Name (#x20 Name)*
[7]     Nmtoken    ::=    (NameChar)+
[8]     Nmtokens    ::=    Nmtoken (#x20 Nmtoken)*

Answer 3

Maybe I'm missing something, but I believe the question is based on a false assumption. In HTML, attributes are strictly defined according to a fixed specification. If you 'make up' your own attribute names, you are no longer writing valid HTML.

Answer 4

It depends what you mean by "allowed". Each tag has a fixed list of attribute names which are valid, and in html they are case insentitive. In one important sense, only these characters in the correct sequence are "allowed".

Another way of looking at it, is what characters will browsers treat as a valid attribute name. The best advice here comes from the parser spec of HTML 5, which can be found here: http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html#before-attribute-name-state

It says all characters except tab, line feed, form feed, space, solidus, greater than sign, quotation mark, apostrophe and equals sign will be treated as part of the attribute name. Personally, I wouldn't attempt pushing the edge cases of this though.