If you’ve ever noticed “that flicker” in WebKit-based browsers while performing certain CSS operations, especially CSS transforms and animations, then you’ve most likely come across the term “hardware acceleration” before.
The will-change property allows you to inform the browser ahead of time of what kinds of changes you are likely to make to an element, so that it can set up the appropriate optimizations before they’re needed, therefore avoiding a non-trivial start-up cost which can have a negative effect on the responsiveness of a page. The elements can be changed and rendered faster, and the page will be able to update snappily, resulting in a smoother experience.