Add aria-required as a supported property of checkbox

Fixes issue #921

index.html

@@ -1571,6 +1571,7 @@ <h2>Definition of Roles</h2>
 							<ul>
 								<li><sref>aria-expanded</sref></li>
 								<li><pref>aria-readonly</pref></li>
+								<li><pref>aria-required</pref></li>
 							</ul>
 						</td>
 					</tr>
@@ -12659,6 +12660,7 @@ <h2>Change Log</h2>
 		<h2>Substantive changes since the last public working draft</h2>
 		<ul>
 			<!-- EdNote: After each WD publish, move contents of this list into the next one below. -->
+			<li>02-May-2019: Add aria-required as a supported property of checkbox</li>
 			<li>02-May-2019: Add code role</li>
 			<li>02-May-2019: Add aria-expanded support to application and checkbox roles.</li>
 			<li>02-May-2019: Remove aria-expanded support from the following roles: alert, alertdialog, article, banner, blockquote, caption, cell, complementary, contentinfo, definition, deletion, dialog, directory, feed, figure, form, grid, group, heading, img, insertion, label, landmark, legend, list, listitem, log, main, marquee, math, menu, menubar, navigation, note, paragraph, radiogroup, region, search, select, status, subscript, superscript, table, tabpanel, term, time, timer, toolbar, tooltip, tree, treegrid.</li>