[mmis1000]

Well, you can actually write slow css if you make real deep nested flex container. And it's not even too rare. You can actually find such example in yhe wild.

The spec of flex layout requires it to layout its child elements several times to compute actual layout. Make it deep and nested without proper constrains will results in n*n*n*n… layout computations and bring down the browser on resize.

[cluckindan]

It’s not even that hard. Use flex for layout and you can instantly see how slow reflow becomes when resizing the window.