fix(getSelector): improve selectors for namespaced elements

- by default, ensure the nodename is escaped
- for XHTML documents, only use the local name

Replaces dequelabs#566
Closes dequelabs#563

lib/core/utils/get-selector.js

@@ -1,4 +1,5 @@
 const escapeSelector = axe.utils.escapeSelector;
+let isXHTML;
 
 function isUncommonClassName (className) {
   return ![
@@ -27,7 +28,7 @@ function getDistinctClassList (elm) {
 }
 
 const commonNodes = [
-  'div', 'span', 'p', 
+  'div', 'span', 'p',
   'b', 'i', 'u', 'strong', 'em',
   'h2', 'h3'
 ];
@@ -79,7 +80,6 @@ const createSelector = {
   // Get uncommon node names
   getUncommonElm (elm, { isCommonElm, isCustomElm, nodeName })  {
     if (!isCommonElm && !isCustomElm) {
-      nodeName = escapeSelector(nodeName);
       // Add [type] if nodeName is an input element
       if (nodeName === 'input' && elm.hasAttribute('type')) {
         nodeName += '[type="' + elm.type + '"]';
@@ -130,7 +130,12 @@ const createSelector = {
  * recognize the element by (IDs, aria roles, custom element names, etc.)
  */
 function getElmFeatures (elm, featureCount) {
-  const nodeName = elm.nodeName.toLowerCase();
+  if (typeof isXHTML === 'undefined') {
+    isXHTML = axe.utils.isXHTML(document);
+  }
+  const nodeName = escapeSelector(isXHTML?
+     elm.localName
+    :elm.nodeName.toLowerCase());
   const classList = Array.from(elm.classList) || [];
   // Collect some props we need to build the selector
   const props = {
@@ -174,7 +179,7 @@ function generateSelector (elm, options, doc) {
   let { isUnique = false } = options;
   const idSelector = createSelector.getElmId(elm);
   const {
-    featureCount = 2, 
+    featureCount = 2,
     minDepth = 1,
     toRoot = false,
     childSelectors = []

test/core/utils/get-selector.js

@@ -194,6 +194,20 @@ describe('axe.utils.getSelector', function () {
 		assert.equal(result[0], node);
 	});
 
+	it('should work on complex namespaced elements', function () {
+		fixture.innerHTML = '<m:math xmlns:m="http://www.w3.org/1998/Math/MathML">' +
+		  '<m:mi>x</m:mi>' +
+		  '<m:annotation-xml encoding="MathML-Content">' +
+		    '<m:ci>x</m:ci>' +
+		  '</m:annotation-xml>' +
+		'</m:math>';
+		var node = fixture.querySelector('m\\:ci');
+		var sel = axe.utils.getSelector(node);
+		var result = document.querySelectorAll(sel);
+		assert.lengthOf(result, 1);
+		assert.equal(result[0], node);
+	});
+
 	it('shouldn\'t fail if the node\'s parentNode doesnt have children, somehow (Firefox bug)', function () {
 		var sel = axe.utils.getSelector({
 			nodeName: 'a',