With the ability to enter almost anything into the HTML Content blocks, and especially the WYSIWYG editor used.. it's very, very easy to break the accessibility of your OPAC. We should look into options for hinting at correct headings level start point in the editor and perhaps add options for adding simple styled blocks that conform to Koha best practices (Like the use of page-sections in intranet blocks for example)
TinyMCE actually have an accessability checker tool.. though it looks like a premium plugin to me.. likely licensed? https://www.tiny.cloud/docs/tinymce/6/a11ychecker/
It's an interesting idea. I've been adding element/attribute filtering to the HTML Content blocks, so the UI for this is certainly on my mind these days...
Maybe out of scope of this bug, but: If the WYSIWYG editor can cause problems with accessibility especially for HTML customizations for the OPAC, would it be better to be able to choose Texteditor / Codemirror specifically only for HTML customizations and WYSIWYG for "everything else"? The way it is currently, system preference AdditionalContentsEditor sets the default editor for all additional contents, so news, HTML customizations, etc. Maybe it would be better to separate the HTML customizations and pages part into a new system preference?
I added a new bug for my previous comment: https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=38218 as it's probably out of scope here :)