Heck, the browser could even do a lot of things that wouldn't even be an option via CSS. For a transition set to occur on :hover, the article suggests that a hu

Heck, the browser could even do a lot of things that wouldn't even be an option via CSS. For a transition set to occur on :hover, the article suggests that a human might want to add a will-change attribute to :hover on the containing element (which could potentially be quite large). But the browser could in principle trigger the equivalent will-change optimizations just for a small buffer region surrounding the element with the transition. Or if you wanted to be really ambitious, the browser could even trigger only when the mouse is nearby and actually moving toward that element. I couldn't do any of that via CSS (and I shouldn't have to be the one to do it via JS).