var stylesheet, breite;

if (window.innerWidth) {
 breite = window.innerWidth;
} else {
 breite = document.body.clientWidth;
}

if (breite <=608) {
 stylesheet = 'position1.css'
}
else if (breite <=760) {
 stylesheet = 'position2.css';
}
else if (breite <=940) {
 stylesheet = 'position3.css';
}
else if (breite <=1200) {
 stylesheet = 'position4.css';
}
else {
 stylesheet = 'position5.css';
}

document.writeln('<style type="text/css"><!-- @import url(' + stylesheet + '); --> </style>');