Some HTML in the FlaggedRevs extension uses the width attribute which is deprecated in HTML5. It should use CSS instead (style="width: 50%;)
$:andre\> pwd /home/user/wikimedia/git/mediawiki-extensions/FlaggedRevs $:andre\> git branch * master $:andre\> grep -r "align=" . $:andre\> grep -r "width=" . ./frontend/FlaggablePageView.php: "<table border='0' width='98%' cellpadding='0' cellspacing='4' class='$tableClass'>" . ./frontend/FlaggablePageView.php: "<td colspan='2' width='50%' style='text-align: center;' class='diff-otitle'><b>" . ./frontend/FlaggablePageView.php: "<td colspan='2' width='50%' style='text-align: center;' class='diff-ntitle'><b>" . ./frontend/FlaggablePageView.php: $table .= "<td width='50%' style='text-align: center;'>"; ./frontend/FlaggablePageView.php: $table .= "</td><td width='50%' style='text-align: center;'>";