Write JavaScript code that is executed during form submission to calculate the cost of each book (based on quantity specified) and the overall total cost. The results should be displayed on the same page beneath the form submission button. Use any CSS at your discretion if desired.
All JavaScript code must be external. If any fields are left blank or do not contain a number, an alert box should display an appropriate error message upon form submission.
Note that your XHTML page must validate as 1.0 Strict - as such, make any needed additions or changes to the XHTML code above.