What server supports HTTP/3 but not HTTP 1.1? The general solution for this is to create a new recipe class that uses a QtWebEngine as the browser. This however is a long project, instead if you need to just do a quick hack, override get_browser() in your recipe to return self. Then implement the open method in your recipe. These will be used by the fetcher to download anything including images.
Code:
def open(self, url):
return read_url(self.storage, url) # here self.storage should be a list created in the __init__ method of your recipe
As for adding headers, there is currently no facility for that, however it should be easy to add.