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Wouldn't this be much more convenient in the form of, say, a Python library that the user can run on their local machine?

You can already do this exact thing with a Python library on the user machine, see here for install instructions: https://gpkit.readthedocs.io/en/latest/installation.html

The intent here is to make it easy for people to get started and quickly write models, without needing to go through installing a set of packages.

There are also services that let you run arbitrary Python code in the browser, e.g. [1].

For sure, but they have a limited range of packages installed, and can be tricky to tune towards a specific need.