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4.1.3 Characters and case states: "All CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent), except for parts that are not under the control of CSS. For example, the case-sensitivity of values of the HTML attributes "id" and "class", of font names, and of URIs lies outside the scope of this specification." Most browsers ignore it if you have used an id with capitals in the HTML but lower case in the CSS, however Internet Explorer will ignore the selector. It would be good if the validator returned at least a warning of a case mismatch.