W3C Test: Import web-platform-tests/html/syntax.

LayoutTests/imported/web-platform-tests/html/syntax/parsing/common.js

+function mark_diffs(expected, actual) {
+  var expected_lines = expected.split("\n");
+  var actual_lines = actual.split("\n");
+
+  var max_length = Math.max(expected_lines.length, actual_lines.length);
+
+  var expected_diff = ["code", {}];
+  var actual_diff = ["code", {}];
+
+  for (var i=0; i<max_length; i++) {
+    if (expected_lines[i] === actual_lines[i]) {
+      expected_diff.push(expected_lines[i] + "\n");
+      actual_diff.push(actual_lines[i] + "\n");
+    } else {
+      if (expected_lines[i]) {
+        expected_diff.push(["span", {style:"color:red"}, expected_lines[i] + "\n"]);
+      }
+      if (actual_lines[i]) {
+        actual_diff.push(["span", {style:"color:red"}, actual_lines[i] + "\n"]);
+      }
+    }
+  }
+  return [expected_diff, actual_diff];
+}