The problem with this approach to assigning meaning to data in files is that the parser is difficult to write and more important it is a custom program. HTML tags that are designed to give instructions on what a file should look like are arbitrarily assigned meta function. Much of the actual data is not rendered because browsers don't know what to do with the arbitrary attribute tags. The meta tag data is lost to the browser user. The same file rewritten in XML will look a lot different.
More meaning is visible.
Now we have a different problem. The display information is missing. The parser knows exactly what each field means and doesn't have a clue about how to display the page. That is the function of XSL, the eXtensible Style Language. An XSL file contains the mapping information to tell the browser how to display the information for a particular class of XML files.