Shadowboxes for IFRAMEs

check out this previous post/topic for additional context:

Seems like that is done on the codepen side?

Make the preview “Click-to-Load” : Check the checkbox next to “use click-to-load” to enable the preview version for your embedded Pen. Preview embeds show a static image preview of your Pen with a “Run Pen” button. When your visitors click the button, the Pen will load.