In the above form, you can enter an arithmetic expression. It will be evaluated when you press the equal ("=") key. It will also convert the result to a fixed value with two decimal places. For instance, type:
10 * .03 =
Once you type the equals key, the text field will be replaced with the value of the expression: "0.30".
If you enter something other than an arithmetic expression, the exact text entered will be retained in the text field. For instance, if you type "one + one =", this exact string will remain.
In Firefox, once you press the equals key, you can use the "Ctrl-z" key press to revert the text field value to the arithmetic expression.