CodePen probably won't work great in this browser. We generally only support the major desktop browsers like Chrome, Firefox, Safari, and Edge. Use this one at your own risk! If you're looking to test things, try looking at Pens/Projects in Debug View.
Click on the floor to start building cubes, click on the faces of cubes to build out from each face. Delete a cube with right-click. Have fun, don't expect the best FPS :)
~ Known oddities in Firefox and Edge.