Animations have been around since before the internet, and making them great is something you could spend a lifetime learning. However, there are some unique constraints and challenges in doing them for the internet.
On a bad Internet connection a lot of sites are refusing to load at all, and those that do have lots of images missing, leaving big blank holes in their web pages. Most of the images, thankfully, aren’t critical to understanding the content, but their absence does remind us that we're waiting for something else to arrive, and in many cases it is perceived performance that is more important than the actual performance itself.
React has changed how we tackle building complex applications. Instead of blindly enforcing DRY or “separation of concerns”, we eagerly co-locate data, logic, & aesthetics with the components that know best so progress can be made, rather than needlessly stifled.