Hey George,
Depending on what HTML Doctype your theme declares, our code is indeed valid HTML. Our code is valid HTML5, but for previous versions of HTML you should expect some “notices” and “warnings” about The Events Calendar use of data attributes and such…
As for accessibility, admittedly there may not be 100% compliance here. 100% is the number we are chasing in terms of compliance with accessibility norms, but at this time there are likely some inconsistencies when The Events Calendar’s output is rendered by screen readers or other similar devices.
I’m sorry that this isn’t a simple “Yep, 100% to both!” sort of reply, but just being candid here and hope the information is clear and helpful. Let me know if this is the case and/or if you have any other questions!
Sincerely,
George