Valid values for id attribute in HTML

For HTML 4, the answer is technically:

ID and NAME tokens 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 (".").

HTML 5 is even more permissive, saying only that an id must contain at least one character and may not contain any space characters.

However, as a practical matter, you will be somewhat more limited if you want your documents to work with a variety of browsers, CSS editors, and JavaScript frameworks.

As noted in other responses, jQuery has problems with ids that contain periods and colons.

A more subtle problem is that some browsers treat id attribute values as case-sensitive, while other browsers do not. That means that if you type id="firstName" in your HTML (lower-case 'f') and #FirstName { color: red } in your CSS (upper-case 'F'), a buggy browser will set the element's color to red. Because both definitions use valid characters for the id, you will receive no error from a validation tool.

You can avoid these problems by strictly sticking to a naming convention. For example, if you limit yourself entirely to lower-case characters and always separate words with either hyphens or underscores (but not both, pick one and never use the other), then you have an easy-to-remember pattern. You will never wonder "was it firstName or FirstName?" because you will always know that you should type first_name