ESPAsyncWebServer VS Svelte

Use esp async Webserver https://github.com/me-no-dev/ESPAsyncWebServer

I don't know what you're doing wrong, but maybe you could change to the ESPAsyncWebServer instead of the fork you're using. Since me-no-dev works at espressif, it seems like that's basically the official webserver to use and I don't see that dvarrel did much good to it in their fork.

For something that's supposed to run at home and not be accessible from the broader Internet, you would do this by having your ESP8266 run as a WiFi client in your home internet and then using something like the EspAsynchWebServer library to make it a mini webserver than can receive requests from any browser in any of your home devices.

There is a library called ESPAsynchWebServer that will easilly allow you to run a webserver in your code on the ESP32 and ESP8266 chips with Arduino and if you check their simple example you can see it's reasonably easy to have the code for doing stuff on the hardware side (say, toggle the pin that controlls an LED) as part of the code that receives and processes HTTP requests.

Check out EspAsyncWebServer, it’s documentation will make clear how it would allow you to do this if you use it.

Go get EspAsynchServer. Check here.

I don't know what kind of Arduino you are using, but I would never try to implement a HTTP server that wasn't asynchronous. Typically using https://github.com/me-no-dev/ESPAsyncWebServer

I would make the ESP32 serve up a web page that displays the battery status, and also handle a button click that turns on or off the relay. You are looking at using https://github.com/me-no-dev/ESPAsyncWebServer

the technical decision how Svelte should treat self-closing html elements was hindered by Prettier:

Svelte

React has introduced measures like batching state updates, background concurrent rendering and memoization to tackle this. My opinion is that the best way to solve the problem is by improving their reactivity model. The app needs to be able to track the code that should be re-run on updating a given state variable and specifically update the UI corresponding to this update. Tools like solid.js and svelte work in this manner. It also eliminates the need for a virtual DOM and diffing.

Similarly to Promises/A+, this effort focuses on aligning the JavaScript ecosystem. If this alignment is successful, then a standard could emerge, based on that experience. Several framework authors are collaborating here on a common model which could back their reactivity core. The current draft is based on design input from the authors/maintainers of Angular, Bubble, Ember, FAST, MobX, Preact, Qwik, RxJS, Solid, Starbeam, Svelte, Vue, Wiz, and more…

Svelte is a powerful web framework that offers a fresh approach to building web applications. Its simplicity, reactivity model, and built-in features make it an excellent choice for developers looking to create efficient and maintainable applications. By following this guide, you should now have a good understanding of how to get started with Svelte and build your first components, routes, and transitions. You can read more about svelte on the official Svelte website.

Use .NET features (especially dotnet watch) as a setup for a client-side Svelte application, starting from a simple C# console app.