Hi, welcome to the community!
The old browser plug-in in 1.9 used NPAPI and ActiveX plug-ins, which were able to run code natively on the user’s computer. Browsers have since dropped support for these plug-ins, so Panda is no longer able to run on the web via these mechanisms.
There is now an experimental new way to run Panda on the web: via WebAssembly. You can see this in action here:
http://rdb.name/panda3d-webgl/editor.html
However, the process to build a Panda app for WebAssembly is still a little involved right now. It’s not straightforward. We’re working on improving it. You can follow the development in this thread.
The signing system was necessary in the old browser plug-ins because Python code ran directly on the user’s computer with no isolation, so we needed a system whereby developers could prove to users that their apps were genuine and not being spoofed by a malicious attacker. But because WebAssembly runs entirely sandboxed (and is safer in that regard), the signing system is no longer necessary.
It’s a similar story for Android: Panda does work on Android, but the experience of deploying a Panda app for Android still needs to be streamlined.