Initial import of ReviewBoardA11yFixes.

This does the following:
* Makes diff file names headings instead of table headers, thus preventing it from being reported as a header for every cell.
* Makes the revision cells normal cells instead of headers, again preventing them from being reported inappropriately as headers for other cells.
* For inserted and replaced lines, prefixes the right hand line number with off-screen text indicating the type of change.

Author: jcsteh

ReviewBoardA11yFixes.user.js

@@ -0,0 +1,53 @@
+// ==UserScript==
+// @name           Review Board Accessibility Fixes
+// @namespace      http://axSGrease.nvaccess.org/
+// @description    Improves the accessibility of Review Board.
+// @author         James Teh <jamie@nvaccess.org>
+// @copyright 2016 NV Access Limited
+// @license GNU General Public License version 2.0
+// @version        2016.1
+// @grant GM_log
+// @include https://reviewboard.*/r/*/diff/*
+// ==/UserScript==
+
+function tweakSideBySide(side) {
+	// Make the diff file name into a heading instead of a table header.
+	// Among other things, this prevents it from being reported as a header for every cell.
+	var elem = side.querySelector('thead th');
+	if (elem) {
+		elem.setAttribute("role", "heading");
+		elem.setAttribute("aria-level", "2");
+	}
+	// Similarly, don't treat the revision cells as headers.
+	for (elem of side.querySelectorAll(".revision-col"))
+		elem.setAttribute("role", "cell");
+
+	// For changed lines, prefix the right hand line number with off-screen text indicating the type of change.
+	for (var tbody of side.querySelectorAll("tbody.insert,tbody.replace")) {
+		for (var th of tbody.querySelectorAll("tr th:nth-child(3)"))
+			th.innerHTML = '<span style="position: absolute; left: -10000px;">' + tbody.className + '</span> ' + th.innerHTML;
+	}
+}
+
+function onNodeAdded(target) {
+	if (target.classList.contains("sidebyside"))
+		tweakSideBySide(target);
+}
+
+var observer = new MutationObserver(function(mutations) {
+	for (var mutation of mutations) {
+		try {
+			if (mutation.type === "childList") {
+				for (var node of mutation.addedNodes) {
+					if (node.nodeType != Node.ELEMENT_NODE)
+						continue;
+					onNodeAdded(node);
+				}
+			}
+		} catch (e) {
+			// Catch exceptions for individual mutations so other mutations are still handled.
+			GM_log("Exception while handling mutation: " + e);
+		}
+	}
+});
+observer.observe(document, {childList: true, subtree: true});