feat(utils): add function isXHTML to test if a document node is XHTML

lib/core/utils/is-xhtml.js

@@ -0,0 +1,16 @@
+
+/**
+ * Determines if a document node is XHTML
+ * @method isXHTML
+ * @memberof axe.utils
+ * @instance
+ * @param {Node} doc a document node
+ * @return {Boolean}
+ */
+axe.utils.isXHTML = function (doc) {
+	'use strict';
+	if (!doc.createElement) {
+		return false;
+	}
+	return doc.createElement('A').localName === 'A';
+};

test/core/utils/is-xhtml.js

@@ -0,0 +1,21 @@
+describe('axe.utils.isXHTML', function () {
+	'use strict';
+
+	it('should be a function', function () {
+		assert.isFunction(axe.utils.isXHTML);
+	});
+
+	it('should return true on any document that is XHTML', function () {
+		var doc = document.implementation.createDocument('http://www.w3.org/1999/xhtml', 'html', null);
+		assert.isTrue(axe.utils.isXHTML(doc));
+	});
+
+	it('should return false on any document that is HTML', function () {
+		var doc = document.implementation.createHTMLDocument('Monkeys');
+		assert.isFalse(axe.utils.isXHTML(doc));
+	});
+
+	it('should return false on any document that is HTML - fixture', function () {
+		assert.isFalse(axe.utils.isXHTML(document));
+	});
+});