Five browser experiments

Testing an iframe’s
postMessage handler

Watch a widget resize. Break the receiver. Add one check at a time.

  1. The happy path
  2. The wrong origin
  3. The right origin. The wrong window.
  4. A billion pixels of valid data
  5. The complete handler

View and edit the source on GitHub

The host runs inside CodePen; the widgets run on GitHub Pages. Open a demo to edit its HTML, CSS, or JavaScript in your CodePen account. The widget source lives in the linked GitHub repository. Save a Pen to update its article embed.