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.
- 1 An empty element is not automatically "NULL" in this sense, but merely an empty string. Some XML applications use additional markup to indicate what empty elements are in fact NULL, if the distinction is necessary.