Welcome to the confusing world of JavaScript file types.
It’s difficult to give a one size fits all approach to this. But there is almost always a way. Unless the library you’re using is really not intended for the browser.
jsdelivr can be hit or miss. It really depends on the library author. Often library authors don’t provide browser versions of their code and in this situation there’s no guarantee that the cdn jsdelivr provides will work in the browser.
Tip: if using a cdn from jsdelivr, or another provider, then check the console for any errors when the app loads. If you see errors then the cdn may not be consumable by browsers.
If using a cdn like this then the namespace will appear on window. Often the namespace used is similar to the library name, but not always. If I can’t find what name the module has used on window I might open the cdn url and try to find it in the source code. 
Alternative approach
I’ve used skypack.dev a fair bit for importing JavaScript. The advantage here is that skypack converts the JavaScript to more modern syntax, which doesn’t rely as much on the library author implementing a browser version. We can import the JavaScript module directly using anvil.js.import_from (I’ve edited the post above to reflect this). Unfortunately the skypack project looks like it’s dead. Last I checked their search feature was broken and the team behind it has moved on to new things.
I now use https://esm.sh/ or https://jspm.org/. These offer the same as skypack.dev.
If you have specific libraries you can’t import into python then pasting the approach you’ve used as well as a link to the original library (either GitHub or the package on npmjs) will allow us to take a look and find a way. Bonus for adding the python code you expect to work.