change background at specific GIF frame

a gif?

There is no way IMO to manage events depending on a current GIF frame.
I advise you to convert your gif frames as a static spritesheet and to take a look at the canvas tag : https://www.williammalone.com/articles/create-html5-canvas-javascript-sprite-animation/
Which is anyway the best and proper way to do what you want (and more).

Thanks very much, I don’t know spritesheet, I’ll have a look.

What exactly is the animation you are trying to show? There are many types of animation these days that are created with a number of different tools.

What @nokaa describes above I do not think will assist your situation. As a spritesheet is for a single animated element.

If you are looking for a complex animation that changes the CSS of the page element then you would probably want to go with GreenSock or a similar JS based Animation library. Javascript is the only way to change CSS on the fly-