What was wrong with E4X?

I’m currently trying to get the E4X back in the browser. What was wrong with it in the first place?

You’re about 7 years behind


This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Warning: E4X is obsolete. It has been disabled by default for webpages (content) in Firefox 17, disabled by default for chrome in Firefox 20, and has been removed in Firefox 21. Use DOMParser/DOMSerializer or a non-native JXON algorithm instead.

Yeah I know that it’s obsolete. But why?

Because there is a better alternative? That’s usually why code becomes obsolete.