Our implementation (and I believe the E4X spec, tho I haven't taken the time to investigate fully) don't properly emit redeclarations for prefixes which are overridden with a different namespace in nested child elements. Two examples: // Example 1 XML.prettyPrinting = false; default xml namespace = "a"; var x = <m/>; default xml namespace = ""; var c = <y/>; x.appendChild(c); print(x.toXMLString()); print(x == XML(x.toXMLString())); Output: <m xmlns="a"><y/></m> false Note that <y/> is missing an | xmlns=""|. // Example 2 var a = <foo xmlns:a="n_a1"><a:bar/></foo>; var b = <a:quux xmlns:a="n_a2"/>; b.appendChild(a.child(0)); print(b.toXMLString()); print(b == XML(b.toXMLString())); Output: <a:quux xmlns:a="n_a2"><a:bar/></a:quux> false Note that <a:bar/> is missing an | xmlns:a="n_a1"|. I'm pretty certain there's at least a bug in E4X's ToXMLString here; I think the condition in step 10(a) is wrong and needs to be changed from: > If there is no ans ∈ AncestorNamespaces, such that ans.uri == ns.uri and > ans.prefix == ns.prefix to: > If there is no ans ∈ AncestorNamespaces, such that ans.uri == ns.uri and > ans.prefix == ns.prefix, or if there exists an ans ∈ AncestorNamespaces, > such that ans.prefix == ns.prefix and ans.uri != ns.uri (In other words, when a prefix is overridden, a new namespace declaration *must* be emitted, because use of that prefix without the redeclaration will use the wrong namespace.) When I tested this by adding the change to the patch in bug 366117, however, it failed to work. I suspect that this is due to |if (!ns->declared) continue;| but haven't stepped through with a debugger to see for sure. Given that both testcases use appendChild, it's possible there may be a bug lurking there as well related to omission of [[InScopeNamespaces]] updates somewhere in [[Put]], but I haven't taken the time to found out for certain.

E4X will be removed again from Spidermonkey (bug 788293)