How can I get around how some browsers(or users can change them) have auto margins already and some browser don't have any when you print a page. I printing invoices and I need to have absolute positions but on some browser it runs off the screen because of the auto margins. Am I going to have to tell users to change this?
Some printers have built in margins so that you can never get an exact positioning with HTML. The best you can do is to keep the width below 750 pixels as that allows for the fixed margins that the printer may have.
If you want to control the position of text more accurately than that you need to use a PDF rather than HTML.