  Automated web accessibility testing tools have been found complementary. The
implication: To catch as many issues as possible, use multiple tools. Doing
this efficiently entails integration costs. Is there a small set of tools that,
together, make additional tools redundant? I approach this problem by comparing
nine comprehensive accessibility testing tools that are amenable to
integration: alfa, axe-core, Continuum, Equal Access, HTML CodeSniffer, Nu Html
Checker, QualWeb, Tenon, and WAVE. I tested 121 web pages of interest to CVS
Health with these tools. Each tool only fractionally duplicated any other tool.
Each discovered numerous issue instances missed by all the others. Thus,
testing with all nine tools was substantially more informative than testing
with any subset.
