Would the Delta Chat e-mail client or the Monocles Chat XMPP client help? They both support webapps-in-chat (WebXDC), and a calendar is one of the many simple apps already created. https://webxdc.org/

Apparently, this should be possible now (and, for apps, the result would be as "usable" as Electron) thanks to WebIDL. For example, Webkit's code to support Javascript access of browser objects is generated from a WebIDL spec. Generating support for <your_language>'s access to Webkit's browser objects is "just work".

Actually, https://tauri.app/ ...

Enough of this tangent now!

I suspect writing cross-platform desktop/mobile apps in HTML/CSS/JS was another big pull in this direction.

Many popular languages are bad, yet JS is the one with a widely-deployed OS interface written in it (WebOS).

If free-software/open-source devs hadn't got caught up chasing all this, there was a chance of replacing JS with other languages in the stack. HTML/CSS/your_language probably for apps initially, even making browsers support plugging in languages later. The docs say anything other than JS is not supported, so no <script type="text/your_language">. If only!

The first fully-developed prototype-based OO language was Self, original Javascript seems quite similar.

As a long-time toe-dipper in functional programming waters, I am yet to appreciate the gestalt of any individual language. Going by Joey Hess of Debian and Git Annex fame, #Haskell is what is most bang for the buck.

I have found, though, that the most useful aspects, in isolation or in combination, are pure functions, immutable data and pattern matching syntax. Maybe you will find them in a language you like. Personally, I am enjoying foraying into #Prolog and #Erlang (or Elixir).