NULL means that an attribute is present but has no value. For example, an empty element in XML1 or None, undef, NULL, or nil in various programming languages.

A different way to indicate that there is no value for an attribute is to not have a placeholder or slot for the value at all, making the attribute "optional".

Null values support View Source

A strong argument for required, but may be empty, elements is to ensure that anybody who does view source can cut and paste have a ready checklist of things to fill in.

CategoryModel, CategorySyntax