I checked my site here https://validator.w3.org/ and have some recommendation about your t4-system-plugin plguin, hope you update your plugin
themes\base\html\layouts\t4\index.php, themes\base\html\layouts\t4\edit.php
Change <style type="text/css"> to <style>
themes\base\html\layouts\t4\element\logo.php, themes\base\html\layouts\t4\layout\offcanvas.php
Change <img class="xxx" alt="yyy" /> to <img class="xxx" alt="yyy">
admin\theme\index.php
Change <link rel="stylesheet" href="style.css" /> to <link rel="stylesheet" href="style.css">
<script type="text/javascript" src="frontend-edit.js"></script> to <script src="frontend-edit.js"></script>
more https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script
Thank you