This T-Shirt wasn't designed as a parody of HTML, but it could be. It is really supposed to be XML. XML is used to be a platform independent, browser independent way to send information. In XML you use the < and > commands to tell the interpretor what kind of info is going to be stored. There is no common type of commands that go in between them like in HTML. For example, you could have this:
That would tell the interpretor that the price is 3.50. You can nest these, like in HTML, so you have categories inside of categories.