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How do you work on elements after an animation has taken place?

I’m trying to mimic the animation from this web design:

So I have a timed animation that happens after the home page has loaded (3s animation with 3 second delay), that slides the main section out of view to the right and changes the background color.

My question is how do I work on the body element AFTER the animation has occurred. Essentially, it’s a new state and I would like to slide up a new main section. Basically, I want this new main section to be out of view, and after the 6s animation, slide into view. I don’t want the user to be able to see the new section with scrolling (in fact, is there a way to not allow scrolling at all on the home page?)

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