The following is an example of using the HTML5 output tag with a form's oninput event to do some calculations on the data within the form. It uses html5Widgets to fix IE9's slightly misbehaved implementation of oninput. Back to the User Agent Man oninput article.

Money You Owe

Enter in the price and we will calculate what you owe us.

The total of the stuff you just bought. When you fill this in, you will see the items below will change.
Taxes: 0
This is how much you have to give to The Man, whether you like it or not.
Total: 0
This is how much you have to shell out in total. Have a nice day!