I did this with an animated GIF. It puts a specific pattern on the display that maximizes the EMI generated by the HDMI cable. You can hear the morse code on an RTL-SDR by tuning to ~300MHz or so.

The animated pattern in the GIF file is practically invisible (only 1 LSB color change). You can set the GIF to the background of your Web site and detect if someone nearby is visiting it that way.

You could use it exfiltrate data that way, it's enough bandwidth for a password or crypto key. A fairly short piece of JavaScript is all that's required to do this. If you keep repeating the message periodically, you can vastly expand the range of the signal, by averaging it at the receiver.