Using Clean theme, view a range of pages in your Moodle site, with as many differnet page layouts as possible. (in particular, some layout files call $OUTPUT->fullheader(), and others call $OUTPUT->navbar, you need at least one of each sort of layout.) For each one, verify that the navbar (often mis-called the breadcrumbs) are in exactly one <nav> tag, not two nested ones. Verify that that nav region is aria-labelled. (NVDA does not seem to support this, so you would need to use JAWS to test this properly, or you could just inspect the HTML.) Repeat this testing using a theme based on base, instead of bootstrapbase.