Beyond not having the basic framework for the HTML, you have one error according to the [W3C validator](http://validator.w3.org/#validate_by_input):

    <input id="display" value="0" size="15" readonly="true" >

`readonly="true"` is not valid, the correct attribute is `readonly="readonly"`

There is a pretty bad UI problem here.  When you click the buttons as follows `9 - 3 = 2`, the calculator reads `62`, whereas it should reset and only read `2` because you clicked the `=` button.