Some native objects are read-only without really seeming to be so (you can write to them but it has no effect). For example, a common advanced javascript is based on extending the Element object by overriding system methods, say, changing Element.prototype.appendChild() to do more than appending a child node - say, initialize it with parent's data. This will fail silently on IE6 - original method will be invoked on new objects instead of the new one.
Some browsers (I don't remember which now) consider newlines between HTML tags to be text nodes, while others don't. So childNodes(n), nextSibling(), firstChild() and the like will behave very differently.
