Context

24 introduced a basic web frontend to use. At the moment, it provides only very bare-bones functionality with minimal styling. I'd expect this to become a bit nicer to look at while not distracting from the overall functionality

A web frontend with a nice, clean looking UI - I don't have a strong opinion what "nice" and "clean" should be, however if you are thinking of contributing and are unsure, feel to post your ideas here and I'll give feedback before you invest too much time
Prioritise function and usability over prettiness - this tool is used by people who quickly need to achieve a task (to convert a json policy into a different format). The design shouldn't distract from this.

My own frontend-skills exist but are rusty - you are free to choose the best tools and frameworks for the job, however I'd prefer minimal learning curves and complexity. In doubt, err on the side of simplicity and well known tools for myself and other contributors to maintain this in the long run
The tool is primarily used by infrastructure people for their work - I assume most will use it on their desktops and laptops. Therefore, responsive design or mobile friendly features are not a priority (however nice to have).

There are several other issues dealing with the web frontend - check the web-frontend label to get an understanding where overlaps might exist.