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.