With https://whereby.com/ you can provide video meetings in the browser. Your attendees just need to visit your custom link to whereby.
You can also share your screen and for small teams (up to 4 attendees) it is gratis.
This is not a code library but a software recommendation. Anyway, i hope this helps.
I now use the Markdown here browser extension for this.
Of course this is not HTML, as mentioned in my question, but it allows the emails to be edited "with code" (Markdown) and do CTRL+ALT+M to have the rendered output.
That's the closest solution I found to what I was looking for. Finally Markdown is even better than editing HTML directly (If you are ...
The Vimium C Web Extension by Dahan Gong has been updated more recently than other similar extensions (it was just updated 4 days ago).
Description from Mozilla's Firefox Add-ons Site (AMO):
Vimium C is an open source browser extension that provides keyboard-based inner-page navigation, browser tab operations, and an enhanced search panel, so you may ...
While I'm still happy with pinboard.in, with raindrop.io a new option has emerged. Advantages over pinboard.in are that there's a free plan (unlimited bookmarks and basic features) and there are mobile apps for all platforms. If I would start from scratch today I would probably choose raindrop.io over pinboard.in.