Hi Julian, just as a comment: I would expect Admin UI developers to use the "Live working with a running Opencast" facility (see https://docs.opencast.org/develop/developer/modules/admin-ui/development/#live-working-with-a-running-opencast)
I see what you're saying, but I have to respectfully disagree with a few points. No hard feelings, though!
For the stylesheets this is *already* true for the Opencast codebase. No browser can interpret Sass.
And last but not least there is this passage from the page you cited:
"Be warned that some functionality in this live setup can be limited. Before reporting an issue, please test if you can reproduced the issue with a built Opencast."
And for me specifically these limitations manifested in the form of crashes after a few clicks. Which might be my fault, I'll admit, but still: All in all I still think we need sourcemaps.