A web engine written in Python.
It's the backbone (Swedish: "ryggrad") of a web browser. Yes, I'll be making my own (almost-)pure-python web browser.
Note
Please note that I do not refuse to use non-python libraries, but I try to avoid it. But I also want to avoid that the project just stalls indefinitely and never gets anywhere useful, not even in many decades, so cryptography, compression and video codecs etc. will probably be handled by existing libraries that don't necessarily need to be written in Python.