Some of the Python standard library is available. It's also possible to use pure Python modules form third parties, provided they only depend on the available parts of the standard library.
It's possible to generate new Python code on the fly. It also means it's possible to implement a REPL in-browser. (There is an example of this on the Brython website.)
Works directly in script tags with minimal setup. Just add Brython from a CDN, and run brython() on page load, and then you're free to use text/python3 script tags with Python code anywhere on the page.
This includes the standard asyncio, time, etc. input() is also awkward. There is an alternative browser.aio, but this means that pure Python libraries depending on those things won't work in Brython.