I used this codepen as a base and modified it to my needs. The author, @fionnachan, had an interesting method I hadn’t thought of: Make a div for each piece of confetti, transform and skew each one, and then set a random offset so it looks like each one moves independently when you apply keyframes animation.
@for in SCSS makes the random offsets and transforms a breeze.
The only downside is that it compiles to almost 4000 lines of CSS. I’m going to try a JS version and see how it performs in comparison.
UPDATE January 7, 2018: I moved to a JS solution based on https://codepen.io/anon/pen/JMOQzE. 50 lines of code + a jQuery dependency that was already getting loaded anyway is much better than 4000 lines of CSS, even if the CSS is novel.