How to make an adaptive triangular background?

Question

css

Good afternoon.
Plz tell me where you can read about how similar triangular shapes are made.
(note that the angle is not acute but with a radius).

Answer 1

why make this up?
it's easier to use png, given that the red block is still a picture

Answer 2

Yuzai PNG. Create two blocks. Red and grey. The red shove PNG is a transparent picture with such cropped borders without color.