What Is Well-Formedness?

Well-formedness is a concept that comes from XML. Technically, it means that a document adheres to certain rigid constraints, such as every start-tag has a matching end-tag, elements begin and end in the same parent element, and every entity reference is defined.
Classic HTML is based on SGML, which allows a lot more leeway than does XML. For example, in HTML and SGML, its perfectly OK to have a
or

tag with no corresponding
and

tags. However, this is no longer allowed in a well-formed document.
Well-formedness ensures that every conforming processor treats the document in the same way at a low level.
This leaves no room for browser interpretation. All modern browsers build the same tree structure from this well-formed markup. They may still differ in which methods they provide in their respective DOMs and in other aspects of behavior, but at least they can agree on whats in the HTML document. Thats a huge step forward.