Accessibility & Readability
Chrome Accessibility Developer Tools are useful for testing for potential accessibility problems in GitLab.
The axe browser extension (available for Firefox and Chrome) is also a handy tool for running audits and getting feedback on markup, CSS and even potentially problematic color usages.
Accessibility best-practices and more in-depth information are available on the Audit Rules page for the Chrome Accessibility Developer Tools. The Awesome Accessibility list is also a useful compilation of accessibility-related material.