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Google's newly-released Material Design Lite:
It's a visual design framework you can use on your own stuff. Think Bootstrap that looks like Google. As you explore the components they make available, you can one-click move them over to CodePen if you want to play with them, customize them, combine them, whatever.
Using Angular? Material Design gets easier and more powerful with Angular Material. The demos in their documentation also offer one-click moves over to CodePen.
We have a big collection of Material Design Pens as well. Not all of these are directly from MDL, but are people's take on the design language.
These integrations are possible thanks to our Prefill API. You POST, we prefill!