Support for custom HTML elements

Hello there.

While messing with the custom JavaScript for my blog, I tried to replace a <div class="wordcloud"> with a HTML Custom Element like <d-wordcloud>. I added the JavaScript for it and everything, and I tried it on a JSFiddle and worked, but it did not work on

Now, I know that custom elements were not mentioned in this supported HTML page, but I figured I would give it a try. Is there any reason that this is not implemented? Could it possibly be added?