Since I validate my extension's pages with the W3C validator, I also see some errors and warnings that should be addressed in the various templates, since HTML 5 compliance is now assumed. Some, like empty headings, it's unrealistic to do anything about. There are these, however, that should be easily addressed:

<script type="text/javascript">

should just be <script>. On the index, these come up too:{{}}

1. Error: The itemtype attribute must not be specified on elements that do not have an itemscope attribute specified.

From line 219, column 107; to line 219, column 224

itemscope><a href="./index.php" itemtype="https://schema.org/Thing" itemprop="item" accesskey="h" data-navbar-reference="index"><i cla

1. Warning: The type attribute is unnecessary for JavaScript resources.

From line 613, column 1; to line 613, column 31

{{ntalk -->↩<script type="text/javascript">↩ va}}

1. Error: An element with role=menuitem must be contained in, or owned by, an element with role=menubar or role=menu.

From line 659, column 4; to line 659, column 88

{{-row">↩ <a class="footer-link" href="./ucp.php?mode=privacy" title="Privacy" role="menuitem">↩ <}}

1. Error: An element with role=menuitem must be contained in, or owned by, an element with role=menubar or role=menu.

From line 663, column 4; to line 663, column 84

{{>↩ |↩ <a class="footer-link" href="./ucp.php?mode=terms" title="Terms" role="menuitem">↩ <}}