What are media queries? 

Responsive design can be accomplished using media queries. How does this work? Think of media queries as a condition that you apply to your CSS. You tell the browser to add or remove certain CSS rules depending on the device or viewport size:

To apply those rules, we will need to use the CSS property @media, as follows:

/* Mobile Styles */
@media only screen and (max-width: 400px) {
  body {
    background-color: #F09A9D; /* Red */
  }
} 

@media only screen and (max-width: 400px) means that if the screen/viewport size is fewer than or equal to 400px, then we apply this CSS. 

There are a few different types of properties you can add to media and target a different type of device.  

For example, you can target especially the iPhone 4 with the following code:

@media only screen 
  and (min-device-width: 320px) 
  and (max-device-width: 480px)
  and (-webkit-min-device-pixel-ratio: 2)
  and (orientation: portrait) {

}

This translates as the following:

and (min-device-width...