Open up the browser dev tools and debug your website.
Extensions are just websites so normal website debugging applies.
You declared a 400 but you didn’t state what was 400ing
I suspect your CSS/JS is whats 400-ing as you used an aboluste link/reference to the JS/CSS in your HTML instead of a relative one as extensions upload to a sub folder.
Or you didn’t state the html to load was index.html in the dev console.
The issue is you’re using absolute paths to files, as Barry previously suggested may be the issue.
Twitch stores Extension files on https://EXTENSIONID.ext-twitch.tv/EXTENSIONID/VERSION/HASH/, but because of the absolute reference it’s attempting to load from the root path https://EXTENSIONID.ext-twitch.tv/.
You will need to use relative paths, ie change /static/css/main.af0a5b9d.css to static/css/main.af0a5b9d.css or ./static/css/main.af0a5b9d.css. This will need to be done for all references to other files, and can usually be handled by configuring your build process accordingly.