What front-end framework does Azure Portal use?

I really love the look of their UI. And it works great. True masterclass of UI design, IMVHO. I particularly respect the tiny cool icons and colors, and the low page memory usage (basically like 60MB in Chrome/Edge, almost as low as a minimal HTML page ~~ for comparison way less than say YT, which can be around 300 MB even before a video plays).

I'm just wondering what framework can produce this output. I took a look at the Elements panel but nothing was recognizable.