Axe-Core is an automated accessibility testing engine implemented as a framework that allows you to “evaluate” the accessibility of web pages or web application screens. This allows it to run automated accessibility tests in various contexts, including a browser, command-line tools, and continuous-integration testing environments.
Axe-Core supports evaluation of web content conformance to WCAG 2.1 (both levels A and AA), as well as checking for common failures (gotchas) and “requiring” you to respect best practices that make life much easier for assistive technology users. The set of rules to test for can be customized, giving the developer enough power and flexibility so as to decide what to test for, and when; all rules supported by axe-core are documented on this page from its documentation.
Axe-Core can be used within browser and Node.JS based environments.