What are valid values for the id attribute in HTML? When creating the id attributes for HTML elements, what rules are there for the value? Answer: 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

Which characters are valid in CSS class names/selectors? What are characters/symbols are allowed within CSS class selectors? I know that the following characters are invalid, but what characters are valid? ~ ! @ $ % ^ & * ( ) + = , . / ' ; : " ? > < [ ] \ { } | ` # Answer:  You can check directly at the CSS grammar. Basically1, a name must begin with an underscore (_), a hyphen (-), or a letter(a–z), followed by any number