The source code of this file show how you can combine normalize.css, jquery mobile, responsive design via media queries and the PHP library Mobile Detect to form a PHP-file that can manage both responsive design and adaptions of code related to the device the HTML-file is displayed on.
Writing HTML that will only be displayed on non-mobile devices.
This <div> will show a different background color depending on what screen size it is displayed at. Check the styles.css-file to see the code.