I apologise if I get some minor details wrong here as I am doing this on a phone and recalling this from memory because I don’t have the time to grab the sources right now.
However… the short version of the plan to solve this that I seem to recall to this:
The Flutter team specifically seemed to indicate that they are already used to operating in non DOM environments where they have to support accessibility across Android, iOS, MacOS, Linux and Windows that doing it on web actually isn’t that big a deal as it first seems.
They already have all of the code in place that builds a full tree (like the DOM) which does a complete mapping between every element (widgets in Flutter lingo) on the canvas to their respective bits of accessibility info. They then just take that tree and hook it up to the respective accessibility APIs that each platform exposes.
At no point have they indicated that this looked like it was going to be a serious roadblock or challenge for them. I believe them and they have a history of this approach working elsewhere.
There is just too much money riding on this investment for them not to get accessibility 100% correct in a web native way.
From there if you are able to expose everything through accessibility APIs then you presumably also have everything you need as a search engine or an adblocker to actively read and modify that canvas. At this point the entire argument that it’s just an opaque set of pixels seems to vanish for me.
This is also AFAIK already a solved problem for them in other products where they are using canvas based rendering such as Google Earth and Google Docs.
I don’t have the details beyond that right now sorry but that passes the sniff test for me at least.
