The current Sparkle site design was made by the great @selcuk, I might have actually never said this publicly which is a failing on my part.
While the site is visually quite impressive (we like it a lot anyway), it’s not super complex to build in Sparkle.
Most of the animated elements use a little bit of scroll animation, such as scaling, rotation or cross fading. Admittedly scroll animation controls are a bit complex to wrap your head around, so it takes a bit of experimentation and trial and error to figure out. For example:
We added some hands on examples recently in the animation documentation:
The only other “weird trick” the home page uses is a black gradient that’s fixed to the browser bottom, which adds a bit of a fade to black effect (and because fix to bottom is a setting of groups, the box with the gradient is grouped with another transparent box):