Pure MQTT is done over TCP which browsers don't support without an extension. I'm trying to keep this purely web based as long as I can :)

Some MQTT servers tunnel messages via websockets, sever-sent-events, and REST calls which are supported by pagenodes.

HA does have a REST API, but with the way PageNodes works you'd have to hardcode the HA password right into the PN workflow. Have you considered adding the equivalent of environment variables, which can be set in a PN account and used as placeholders in workflows?

That's a good question. Our storage is in local indexeddb. And the site is https, so no one should see your flow if you don't share it. That said there's nothing stopping you from reaching out to another secure service or plugin before making requests.

Am I interpreting it right that Pagenodes basically aims to be node-red, but in the browser?

The goals are similar. PageNodes does its best to leverage newer browser capabilites. WebRTC, WebUSB, SeviceWorker, offline support, etc.

Ok, so less "prototype in the browser, then offload to a server once it works", but more for local "app" type things? Interesting idea, has some limitations but also opens up tons of interactions that are harder for a server-based solution (webcam, ...)

Bingo! It's always evolving too. We do use a lot of experimental flags from the browser, this helps work with up and coming features as well for learning about new APIs very easily.

You could use Octoblu Meshblu node in PageNode to connect Jarvis to an MQTT platform.

Octoblu confuses me. I can't find anything about their pricing scheme (if any), while all their professional partners listed make me think there definitely has to be one hidden in there somewhere.

I don't believe there is any pricing scheme at all as of yet. It's a great opportunity really, check it out!