CSS: Background, CSS Sprites Tutorial

What's a CSS sprite?

In many websites, it needs lots of icons. For example, here's icons used by Google Plus website.

Normally, each icon is a image file. For each image file, you need a HTTP request. Too many HTTP request makes your site loading slow. (regardless of the internet bandwidth, because each HTTP request usually means a TCP connection, and starting TCP connection has latency. 〔➤ Computer Networking, TCP/IP Basics〕)

CSS sprites is one way to solve the many http image request problem. Instead of using many images, you combine them into one big image, and use CSS to show part of the image, the icon you want.

(Note: another solution is using CSS: Data URI Scheme, but CSS sprites is in general more efficient, when your sprites are larger (say, bigger than 16×16 pixels, or you have more than 10 icons.).)

(Note: CSS sprite is often used by highly trafficked website that needs tens of icons. It also reduces server load. For small sites, you don't need to use it.)

<div class="flameicon"></div>

.flameicon {
width:17px;
height:22px;
background-image: url(i/Google_Plus_css_sprites.png);
background-position: -18px -20px;
}

How to find the “background-position” for a sprite icon?

Suppose this is your image:

Suppose you want the red flame icon.

① Find the coordinates of the upper-left corner and bottom-right corner of the icon you want.

For the flame icon, its upper-left corner is (18,20) and bottom-right corner is (35,42).

② The offset for the icon would be background-position: -18px -20px.

The width and height for the element would be: 17px and 22px. (derived by: 35-18=17, 42-20=22.)