if (InternetExplorer)  { document.write('<link rel="stylesheet" href="test_ie.css" type="text/css">'); }
if (FireFox)           { document.write('<link rel="stylesheet" href="test_ff.css" type="text/css">'); }
if (Opera)             { document.write('<link rel="stylesheet" href="test_op.css" type="text/css">'); }
if (Onbekend)          { document.write('<link rel="stylesheet" href="test.css" type="text/css">'); }