What is the plan for defining accessibility on advanced custom widgets?
The current plan is for ARIA to enable custom widgets. However, this is slated for a future version of ARIA. If you’re interested in helping to shape the development of this part of the ARIA standard, please take a look at the JSON ARIA proposal. Alternatively, if XBL becomes ubiquitous on the web, it would clearly be the natural approach. However, it is unlikely that XBL will be adopted by all browsers in any reasonable time frame.