Fix #12313, .height()/.width() just use CSS if no offsetWidth. Close g…

…h-909.

src/css.js

@@ -411,7 +411,10 @@ function getWidthOrHeight( elem, name, extra ) {
 		valueIsBorderBox = true,
 		isBorderBox = jQuery.support.boxSizing && jQuery.css( elem, "boxSizing" ) === "border-box";
 
-	if ( val <= 0 ) {
+	// some non-html elements return undefined for offsetWidth, so check for null/undefined
+	// svg - https://bugzilla.mozilla.org/show_bug.cgi?id=649285
+	// MathML - https://bugzilla.mozilla.org/show_bug.cgi?id=491668
+	if ( val <= 0 || val == null ) {
 		// Fall back to computed then uncomputed css if necessary
 		val = curCSS( elem, name );
 		if ( val < 0 || val == null ) {