Seriously though, sending PJL commands directly to printers on port 9100 is such a neat hack. Back when I was working help desk in college, much fun was had the day I discovered you could change the "READY" string on HP printer displays to anything you wanted. "INSERT COIN", "CHECK ENGINE", etc.
In the shared link you can find a demo that I made in 2011 that allows you to print directly to many local network printers. I tried it today, and it still work in both Firefox and Chrome. It uses an XMLHttpRequest to send a request to printers on port 9100. Most printers will receive the HTTP request and parse it as a raw print job. In normal cases, the printer will just print the HTTP headers, however I use some control characters to escape to the PJL (print job language) world, where I reset the page by drawing a blank box over the headers and move the cursor to the top left. Besides PJL, it is possible to send postscript data as well.
This is extremely scary, as most printers allow you to perform firmware updates and more over PJL. But, as the Chromium team told me in 2011, this WONTFIX as it is the fault of your printer for being insecure.
