Jars,
What you see in the browser is one thing. It is the result of the
browser parsing the html source of the page. Since the image map
itself is not a visible element, you dont see it. However if you check
the source (Ctrl + U in firefox, or right click "Ver codigo fonte"),
you see that the actual value of $_POST['html_container'] IS there.
If you want to see it without opening the source view, you should use
print_r($_POST)
in your php code.
Espero que eso va ajudar.