Testing with http://people.mozilla.org/~cmccormack/tests/interface-object-property.html, I find that Opera, Firefox and IE allow the property to be deleted, while Safari reports the property as non-configurable so refuses to delete it and Chrome reports the property as configurable but still refuses to delete it.

Do you have a test case showing interface object properties being deleted in Opera/Firefox/IE?

OK, looks like the spec is correct in the general case. However, something goes wrong with window.Node in particular in Gecko; I'm assuming that's a Gecko bug. (See <http://software.hixie.ch/utilities/js/live-dom-viewer/saved/1120>.)

I'm satisfied with your response.

(And for people who are interested: <http://w3c-test.org/webapps/DOMCore/tests/submissions/Ms2ger/interfaces.html>.)

Here is your code:

function testInterfaceDeletable(iface) {
  test(function() {
    assert_true(!!window[iface], "Interface should exist.")
    assert_true(delete window[iface], "The delete operator should return true.")
    assert_equals(window[iface], undefined, "Interface should be gone.")
}
 Avoid getting bit by ASI bugs by consistently using explicit semicolons where they are required.

The delete operator does not "return" anything. The delete operator is just what it is: an operator. It performs an operation which has a boolean result. 

test(function() {
  for (var p in window) {
    interfaces.forEach(function(i) {
      assert_not_equals(p, i)
    })
  }
}, "Interface objects properties should not be Enumerable")

If `window` is specified to have own interface objects as own properties, then you can use:

({}).propertyIsEnumerable.call(window, "Node");

Otherwise, if interface objects can exist in `window` prototype chain, `propertyIsEnumerable`, which doesn't check the object's prototype, won't work there.

Does WebIDL or HTML5 specify `window` to have `Object.prototype` on its prototype chain? If so, then the code could use `window.propertyIsEnumerable("Node")`.