null
Mark Baker: Here's how I see it. HTTP is the protocol, and no other protocols are required in order to get stuff done. Perhaps Mark has a narrower definition of communcations protocol than I do? HTTP does describe how hypertext data is transferred, but not how it is formatted. HTML is one such format. For example, one would not have hypertext without something like anchor elements. In XHTML, this is defined a bit more formally.
Since Mark referred to my neurotransmitters essay, let me provide a bit more background. While we as an industry have been successful at building "single celled" computers ranging in size from embedded devices to supercomputers, the internet is a different beast: a multi-celled computer. At some point nature stops building progressively bigger cells and takes a different approach. Something that is qualitatively different. One in which cells have state and communicate via messages. Glucose causes the beta cells of the pancreas to emit insulin.
WSDL is a means of more formally describing the association of such input messages with output messages.