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...