HTML rendered via what? nw.js? Electron? I feel like the former is a bit more solid by virtue of having been around longer, but unfortunately both ecosystems are such a mess a11y-wise that I haven't really delved into them. In any case, it's probably a safer bet than something like GTK/QT which is either entirely or mostly inaccessible. But HTML rendered in a browser at least has the potential to be highly accessible, and one workaround for poor a11y implementation of nw.js/Electron would be exposing the HTML to another browser over a local port with functions callable via JS over the server port.