This has been a rough saga with WebIO, but I think trying to automatically install the extensions is a good idea.
There are however some issues:
The installation does not respect the active Julia project (so if the frontend/Julia are using different version of WebIO, bad things might happen)
The installation of the notebook extension is global (whereas the JupyterLab installation is only against the Jupyter installed by Conda.jl - this is a limitation of how notebook extensions work)
This has been a rough saga with WebIO, but I think trying to automatically install the extensions is a good idea.
There are however some issues: