A "test page" as you describe could be very helpful - I think even our devs would enjoy that 🙂
I think its worth mentioning for anyone reading that is new to CSS editing, that browsers have very robust tools for live-editing CSS.
I walk through a number of techniques in a video here:
(it shows off an older iteration of Kagi design, but the techniques all still apply)
While there's few queries that show off every element, I would imagine for many edits you can get a large portion of the work done using live editing, without needing to reload at all. Then simply drop in your edited CSS from the Sources tab - I show that off at this timestamp.