Currently the outlook seems poor.
As I understand it, the current solutions are either Chromium-specific or Firefox-specific, with no Safari support whatsoever. I'm not interested in adding features that can only work in bleeding-edge browsers, especially if that browser is Chrome.
Worse, most new web APIs are gated behind the requirement for a "secure context", which demands that the page itself be served via HTTPS and not accessed from a local filesystem. This severely limits what I can use in a standalone single-document HTML build of Decker.
If, in the future, a portable API becomes broadly available for saving in place, I'd be delighted to add support.