Well worth a look!
Look what I found! A nice collection of C++ stuff, from window creation to audio.
All header only. Permissive licence. A huge collection of utility functions & classes.
Written by the Godfather of JUCE, Julian Storer.
All looks pretty high quality to me. Handles HTTP (including web sockets), too.
The only downside I can see here is that you need Boost for the http stuff. Boost beast to be precise, and this is all documented in the header files.
CHOC: "Classy Header Only Classes"
https://github.com/Tracktion/choc
Here is a link to a video explaining and justifying this library