With Brython you can write browser programs in Python instead of Javascript, by inserting Python code in an HTML page by:

<script type="text/python">
...
</script>

Usage:

pip install brython

Then in an empty folder:

python -m brython --install

or in a folder with older versions already present:

python -m brython --update

The package includes a page demo.html with examples of use. For more information see the Brython site.