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UPDATE: I'm currently in the process of updating the article, as my assumptions about the inner workings of WebKit are incorrect. I will update this article with the relevant facts and provide concrete information in place of my guesstimates below.
I've recently stumbled upon an interesting discovery regarding image rendering performance in most WebKit browsers. Namely, I've been developing a sprite animation component to implement a GIF animation replacement with better compression and performance, where I noticed that some animations appeared to be janky when using multi-frame spritesheets and clipping rectangles. Here's what I found out.
But first, a quick rundown of the basic functioning of the WebKit engine as I understand it.