24 ways: Transitional vs. Strict Markup: "Transitional vs. Strict Markup
When promoting web standards, standardistas often talk about XHTML as being more strict than HTML. In a sense it is, since it requires well-formedness and quoted attribute values. But there are two flavours of XHTML 1.0, defined by the Transitional and Strict DOCTYPEs. And HTML 4.01 also comes in those flavours."
Useful little comparison