Media Query is a condition declared for browser to include or exclude a set of CSS rules or a separate stylesheet. Read more about Media Queries.
There are two methods for CSS declaration with Media Queries:
1. Declare a single stylesheet and include Media Queries into CSS. Here is an example:
/*---rules for mobile devices----*/
@media screen and (max-width: 480px)
{
#wrapper{
width: 320px;
min-width: 320px;
}