What I need to do is something like an auto-start transition. But the transitions can't start automatically, so I thought to utilize an animation that not return to the original CSS style of an element after the end of the animation.
animation-fill-mode: forwards
is what you're looking for. This makes the element keep the values from the last keyframe of the animation.
More documentation here: https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode