A student reports this problem when using HTML: “When building my home page, I couldn’t get the background color to change until I put a # in front of the value. Now it works.” What kind of error is...

A student reports this problem when using HTML: “When building my home page, I couldn’t get the background color to change until I put a # in front of the value. Now it works.” What kind of error is it? What rules were applied, and which should have been applied earlier?