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Is this the preferred way to do animations and transitions?

So I have some transitions I’d like to apply when a user scrolls an element into the viewport.

My plan is to grab the bounding corrdinates, find the element’s inner width, and test if the corrdinates match.

If true, add the class for the changes.

I’m a bit stumped on how I’d add the class though. Would changing the opacity in a class and then using ClassList.add(class) be enough to make it change?