max-width

...is the opposite of min-width so to speak in media queries. It does exactly what you think it might do.

@media (max-width: 600px) {
   .whatever {

 
 

This suggests that max-device-width (in contrast to max-width) would measure the width of the device itself (and not the active area within the browser) and that it, yes, it does not react to browser resizing. Lame.