Nice writeup by Josh Comeau on how he created the on-scroll-animated bézier curve on his website
Did you notice that as you started scrolling on this page, the Bézier curves that border the green title hero thingy started flattening? Keep your eye on the swoopy curves just above the post text as you scroll through the top of the document. Notice how they become flat as they approach the header at the top of the viewport?
In a delightful bit of serendipity, I realized while building the blog that this feature would make a great first blog post!
Love the interactive examples embedded into the post. They really help convey the message.