feat(rule): add color-contrast check for unicode characters.

Adding two new config flags to color-contrast so it can check unicode character based icons.

Needs tests.

Related to dequelabs#1906.

lib/checks/color/color-contrast.js

@@ -4,6 +4,18 @@ if (!dom.isVisible(node, false)) {
 	return true;
 }
 
+const visibleText = text.visibleVirtual(virtualNode, false, true);
+const ignoreUnicode = !!(options || {}).ignoreUnicode;
+const textContainsOnlyUnicode = !text.removeUnicode(visibleText, {
+	emoji: false,
+	nonBmp: true,
+	punctuations: false
+}).length;
+
+if (textContainsOnlyUnicode && ignoreUnicode) {
+	return true;
+}
+
 const noScroll = !!(options || {}).noScroll;
 const bgNodes = [];
 const bgColor = color.getBackgroundColor(node, bgNodes, noScroll);
@@ -28,11 +40,11 @@ if (bgColor === null) {
 }
 
 const equalRatio = truncatedResult === 1;
-const shortTextContent =
-	text.visibleVirtual(virtualNode, false, true).length === 1;
+const shortTextContent = visibleText.length === 1;
+const ignoreLength = !!(options || {}).ignoreLength;
 if (equalRatio) {
 	missing = color.incompleteData.set('bgColor', 'equalRatio');
-} else if (shortTextContent) {
+} else if (shortTextContent && !ignoreLength) {
 	// Check that the text content is a single character long
 	missing = 'shortTextContent';
 }
@@ -55,7 +67,7 @@ if (
 	fgColor === null ||
 	bgColor === null ||
 	equalRatio ||
-	(shortTextContent && !cr.isValid)
+	(shortTextContent && !ignoreLength && !cr.isValid)
 ) {
 	missing = null;
 	color.incompleteData.clear();

lib/checks/color/color-contrast.json

@@ -3,6 +3,11 @@
 	"evaluate": "color-contrast.js",
 	"metadata": {
 		"impact": "serious",
+		"options": {
+			"noScroll": false,
+			"ignoreUnicode": true,
+			"ignoreLength": false
+		},
 		"messages": {
 			"pass": "Element has sufficient color contrast of {{=it.data.contrastRatio}}",
 			"fail": "Element has insufficient color contrast of {{=it.data.contrastRatio}} (foreground color: {{=it.data.fgColor}}, background color: {{=it.data.bgColor}}, font size: {{=it.data.fontSize}}, font weight: {{=it.data.fontWeight}}). Expected contrast ratio of {{=it.data.expectedContrastRatio}}",

lib/rules/color-contrast-matches.js

@@ -98,7 +98,7 @@ if (
 	visibleText === '' ||
 	axe.commons.text.removeUnicode(visibleText, {
 		emoji: true,
-		nonBmp: true,
+		nonBmp: false,
 		punctuations: true
 	}) === ''
 ) {

lib/rules/color-contrast.json

@@ -3,7 +3,9 @@
 	"matches": "color-contrast-matches.js",
 	"excludeHidden": false,
 	"options": {
-		"noScroll": false
+		"noScroll": false,
+		"ignoreUnicode": true,
+		"ignoreLength": false
 	},
 	"tags": ["cat.color", "wcag2aa", "wcag143"],
 	"metadata": {