Powering the accessibility of many big companies over the world, Tenon.io is a commercial web service that allows you to test your web page or application for accessibility issues. For each detected issue, it shows where in the code it occurs, possible fixes, and pointer to further information to help you understand what the problem is. An API to integrate the service within more complex workflows is also provided.
While the service is commercial, a limited trial version is available; the company also provides additional commercial services (consultation, issue remediation, etc.).