His code is pretty damn awesome. Open the site in chrome. Open web inspector.

Paste this into the console:

levels.push({ name: 'super secret-1', instructions: 'This is a secret level you get no instructions.', board: 'rggggyyyyy', style: {'flex-direction': 'column-reverse', 'flex-wrap': 'wrap-reverse', 'align-content': 'space-between', 'justify-content': 'center'}, before: "#pond {\n display: flex;\n", after: "}", });

Proceed to the secret 25th level! You can create your own levels, and make a pull request to his git repository if you really want to add some interesting practice levels. Just modify levels.js.