Gives feedback to authors about the accessibility problems of their posts and pages.
The guidelines are inherited from the QUAIL library. You can view all the tests that QUAIL provides, along with their alignment to each guideline at http://quail-lib.org/tests.
Tests are broken into three levels -- called severity levels -- that define how accurate a test is, and therefore, how severe errors the test finds are. They are really a level of how sure we are that there is a problem on a page. All accessibility errors should be fixed, but some things are not possible to test through automated processes alone (like if an "alt" text of an image is correct.)