1. Anyone producing an ebook for public consumption should adopt the ePub3 format.

Having evolved from early ebook formats, ePub3 is now virtually the international standard accepted by every eBook distributor.

More on Kindle's policies on Accessible ePub3

2. To gain universal acceptance, add "Accessibility" to the ePub3.

Accessibility considerations have evolved into web standards over the past 20 years. 

Since ePubs are essentially a collection of web technologies, the accessibility considerations for websites also apply to EPUB publications.

More on standards

3. Accessibility is becoming mandatory.

See the value added by building accessibility into your ebook.