What are valid values for the id attribute in HTML?

Question

When creating the `id` attributes for HTML elements, what rules are there for the value?

Answer 1

That depends on what version of HTML you are creating. For this answer HTML 4 and 5 are really the only valid concerns.

HTML 4:

ID 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:

ID tokens must contain at least one character, and must not contain whitespace, such as spaces, tabs, and new line characters.

Universally:

The ID attribute can be used on any HTML element.

The ID token is case-sensitive.