<html>
<head> <style type="text/css"> only screen and (max-width:1200px){ #test{ font-size: 100; } } only screen and (max-width: 800px) { #test{ font-size: 50; } } </style> </head> <body> <div id="test"> <div style="height:2em;background-color:red"/> </div> </body></html>
and后面要有空格