My team at Microsoft maintains an open source browser extension for this called Accessibility Insights (https://accessibilityinsights.io), which is what Microsoft recommends internally for its own teams developing websites.

It offers a fast 5 minute scan for some of the most common/easily detectable issues, and also a much fuller assessment with a mix of automated checks and guided/assisted manual tests that aims to help web developers who aren't accessibility experts achieve full WCAG 2.1 AA compliance.

Happy to answer any questions about it!