Detect if a browser in a mobile device (iOS/Android phone/tablet) is used

Is there a way to detect if a handheld browser is used (iOS/Android phone/tablet)?

I tried this with the goal to make an element half as wide in a browser on a handheld device but it doesn't make a difference.

width: 600px;
@media handheld { width: 300px; }

Can it be done and if so how?

edit: From the referred page in jmaes' answer I used

@media only screen and (max-device-width: 480px).