CodePen is a social development environment for front-end designers and developers. You can build and deploy a website, show off your work, build test cases to learn and debug, and find inspiration. This is a tip shared by Cosmin on emailgeeks Slack channel:
Small tip for sharing HTML email CodePens: append a ?editors=1000 to the URL so that the CSS/JS panes collapse automatically.
Example
https://codepen.io/Wiredmessenger/pen/pwNPmz?editors=1000