You do get a lot for free with the DOM, but it's messy, works differently between different browser and you have to constantly fight it. With your own rendering engine it's more work but you have much more freedom and can get better performance.
AFAIK it applies to everyone. But you don't get automatically fined, someone has to sue first. Maybe there has to be a business relation between the user and the web site? And yes, with HTML you need to still use all the right tags and attributes to make it accessible. Firefox has a nice accessibility tree view in its dev tools that you can use for debugging. Is there something like that for this toolkit? Is it somehow exposing an accessibility tree? Replicating everything in DOM for accessibility?
With DOM you get accessibility integration. At least in the EU if you provide a web service you have to be accessible, otherwise you can get sued.