Checked the accessibility related checklist items across the top three most visited guides for each person in Research and Learning, here are some of the most common issues that came up
I think this one is obvious. Links to webpages get moved around or taken down from external sites.
"here" and "form" were the most common vague names for links. Easy fix is to state where "here" is in the link name and to add the form name to the link name
Decorative and heading images often lacked alt text. If it's a decorative image, describe why that image was chosen and how it fits in the theme. This allows those using screen readers to appreciate your intentionality.
Most frequently, people skipped H3. Headings are the structural organization of your LibGuides. Use these to denote where in the outline of the content people are. If you use H3, but don't like the visual aspect, you can change the font size while keeping that html tag.
Headings allow the reader to skim and quickly find the information they're looking for. Remember Ranganathan's "Save the Time of the Reader" Law of Library Science? This is a great application of that.
Using the HTML tag for your heading structure allow those with screen readers to skim your guide as well. Skipping a heading level creates confusion, and using a heading to denote emphasis implies structure and more content than there is.