Examples: why you shouldn't use obsolete JavaScript

Many JavaScript properties and methods are now marked deprecated, that is, you shouldn't use them any more.
Now in many cases, these features are still usable; they still do what they were originally doing. But they were deprecated for a reason, and giving them up and using the recommended features instead should be a top priority.
If you're not convinced, here are some features you shouldn't use and the reasons why.

The all collection

Originally only implemented in IE, the all collection has been built into other browsers as well, leading developers to think that it's OK to use. However, there are edge cases where the other browsers disagree with IE.

Given this JS:

  var theAll = document.body.all; console.log(theAll[0].tagName);

In IE, all is defined on all DOM elements, in other browsers only on document. So this will fail everywhere but in IE.

Also, document.all['keyword'] will find elements with name="keyword" in most browsers, but will only find id="keyword" in Konqueror.