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CascadiaCSS, "A CSS conference for the Pacific Northwest," just wrapped up earlier this month. I SO WISH I was there, because they had one of the coolest ideas ever: CSS Layer Tennis with CodePen!
Layer Tennis was invented by Coudal and typically involves Adobe Creative Cloud stuff and passing files back and forth. Moving that concept to CSS is a no brainer, especially with a tool like CodePen where you can just fork the previous Pen.