The HTML spec says

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 (.).

And even though the SGML declaration of HTML 4 uses the value 65536 for NAMELEN, it notes Avoid fixed limits.

But surely browsers, CSS implementations, and JavaScript toolkits must have some limits on the length they support. What is the smallest such limit that is safe to use in a HTML/CSS/JS application?