Require that, at least, document.body exists before firing the ready event. This may stop jQuery from working on pages that don't create a document.body (such as Safari, when no body exists) but considering that we've required a document.body since 1.3 and haven't broken any pages, this doesn't appear to be a pressing issue. Used the test case written by dmethvin to verify the bug and the fix. Fixes #4320.

src/event.js

@@ -791,6 +791,10 @@ jQuery.extend({
 	ready: function() {
 		// Make sure that the DOM is not already loaded
 		if ( !jQuery.isReady ) {
+			if ( !document.body ) {
+				return setTimeout( jQuery.ready, 13 );
+			}
+
 			// Remember that the DOM is ready
 			jQuery.isReady = true;