How to make such a moving object using CSS?

Question

css

Hi all!
I’m trying to implement the same clouds as on this site in the first block https://www.xeovo.com
I didn’t manage to steal, so I’m trying to find an approximate article where they describe how to do this.
The most interesting thing is that even on mobile everything displays normally, I won’t show old sketches because there is horror.
I would be very grateful if you help.

Answer 1

css animation

@-webkit-keyframes fadeInDownBig {
  0% {
    opacity: 0;
    -webkit-transform: translateY(-2000px);
    transform: translateY(-2000px);
  }

  100% {
    opacity: 1;
    -webkit-transform: translateY(0);
    transform: translateY(0);
  }
}