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Can XHTML and HTML class attributes value start with a number?

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No. They have to be SGML names. They "must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

However, class names that start with a number are supported by IE.

EDIT: meder pointed out that you can use Unicode characters and they seem to work in all browsers. I don't know if it complies with the specifications, but it does seem to validate. See http://css-tricks.com/unicode-class-names/ and http://snook.ca/archives/html_and_css/unicode_for_css_class_names

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Your quote refers to ID and NAME tokens. But the value of the class attribute is CDATA.
@Šime Vidas - I think class is a CDATA list of NAME tokens
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No, they cannot. They must begin with a letter. Some browsers may erroneously support them, though.

EDIT: You can start off with unicode escape points and specify the code for a number.

EDIT #2: Test case http://work.arounds.org/sandbox/66/run

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You can specify numbers using hex code points, .\34 is the UTF-8 code for number 4 per yoursiteisvalid.com/validnews/…
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From the CSS spec:

"In CSS3, identifiers (including element names, classes, and IDs in selectors (see [SELECT] [or is this still true])) can contain only the characters [A-Za-z0-9] and ISO 10646 characters 161 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit or a hyphen followed by a digit. They can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B\&W\?" or "B\26 W\3F". (See [UNICODE310] and [ISO10646].)"

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2018: this link is broken / incorrect. Quoted text appears nowhere at the linked page, and the # ref is broken. I suspect the text was taken from a draft that embedded CSS2 and was later updated, so that the correct link today (becaues I needed to verify ths myself in updating an old parser!) is: w3.org/TR/CSS2/syndata.html#characters

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