Martin Fowler’s definition of refactoring is “a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing its observable behvior”. How does this apply to web sites? If you convert a table-based layout to a CSS layout, and the appearance doesn’t change, are you changing the behaviour? Most users would say no. A blind user with a screenreader may think differently.
“Refactoring HTML” is also the title of a book that Elliotte Rusty Harold is working on which I’m looking forward to seeing.