Web accessibility has been growing a lot recently, in part due to the US Department of Justice pursuing violations of
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That being said, I don't think the market for human testers is very big. A lot of companies are selling automated testing products. I can't speak for the process those companies are using to develop the systems or their testing process.
Many organizations are starting to hire people specifically to keep them compliant with regulations. If an organization accepts federal funding, they need to be compliant, so you see a lot of higher education jobs for web accessibility coordinators or something similar. So if you want, instead of testing for accessibility, you can develop for it. This would definitely involve a lot of testing, but you also need to know how to find the solution.