Our own Sarah Drasner:
Unlike fonts, colors, and so on, we tend to add animation in as a last step, which leads to disorganized implementations that lack overall cohesion. If you asked a designer or developer if they would create a mockup or build a UI without knowing the fonts they were working with, they would dislike the idea. Not knowing the building blocks they’re working with means that the design can fall apart or the development can break with something so fundamental left out at the start. Good animation works the same way.
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Animation in Design Systems is a post from CSS-Tricks