What's stopping me from using arbitrary tags in HTML?

Question

Even the new HTML5 tags aren\'t enough to describe structures without falling back to divs. What\'s stopping me from changing:

Answer 1

getElementsByTagName fails me with custom tags. Example,

    
x
    
y
    
z

This fails with IE8 (Quirks Mode or Standard Mode)

var mytag = document.getElementById('mytag'); // it's found
var mydivs = mytag.getElementsByTagName ('div'); // but this is always 0

Unless your html tag reads

...