I've been following the Stellarium project for upwards of a decade and I'm always pleasantly surprised at how far it's come, even if I still have a few grips about telescope integration (still no built-in ASCOM or INDI interfaces).

I did some work for a company about 2 years ago to port a stripped-down version of Stellarium for internal-use on some custom ARM-based in-field devices (the company worked in the satcom industry) and I'm still amazed at the amount of complexity that goes into astronomical simulation. In fact, I recommend devs interested in this space check out the Stellarium source repo, which provides ample amounts of well-designed (Qt-ish) C++ surrounding astronomical projection, star rendering, ephemeris calculation, etc.

While this online-version doesn't seem to share too much with the regular desktop Stellarium (WebAssembly port, anyone?) it does appear to be one of the best web-based planetariums currently.