The XML namespace attribute is mandatory in:

Which element cannot go in the tag

How is the default namespace usually included on an XHTML document?

Which of these, by default, is a block-level element?

Correct formatting for including JavaScript?

How many parsing modes does XHTML have?

What is a correct document type declaration for XHTML?

Difference between XHTML and HTML regarding whitespace?

Why is getting the MIME type correct important?

What happens if you do not include the XHTML default namespace in the declaration?