If you are talking about a browser based POS app then it basically can't be done out of the box. There are a number of alternatives.
- Use an applet like Scott Selby says
- Print from the server. If this is a
cloud server, ie not connectable to the receipt printer then what
you can do is
- From the server generate it as a pdf which can be made to popup a print dialog in the browser
- Use something like Google Cloud Print which will allow connecting printers to a cloud service