I must mention this resource as it is a guarantee for a high-quality content with no hidden agendas (free of charge).
When aspirants learn about other web technologies – they should not forget about the accessibility and when more experienced developers search for resources they should also not forget to implement the accessibility.
At the same time Firefox is also investing in it’s own developer tooling and working towards accessibility tooling as well. It is an value added appendix too.
Mozilla Developer Network – Accessibility section (opens in new window).