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Mastering CSS3 Multiple Backgrounds

Konstantin Kichinsky
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Our journey to the world of CSS3 shadows continues. Today we will focus on another interesting feature—how to use multiple backgrounds with CSS3.

Backgrounds Composition

There are many reasons why you may need to create a composition of multiple images to build your background. I think the most important are the following:

  • to reduce bandwidth usage where the sum of the file sizes of separate images is less than the size of an image with merged layers (especially if your image contains repeating patterns), and
  • to provide a way for independent manipulation of different layers (for example, if you are going to implement a parallax effect).

I believe you may have other reasonable arguments. :)

Classic Approach

So we need to build a multi-layered background by placing some images on top of others. How is this problem usually solved? It is really easy: just create a container (like a div element) for each of the images you have and add a background for it using a CSS rule. Next, you insert one container into another, or place them in a row, and apply corresponding CSS positioning rules. I have here a simple sample:

<div class="sample1">
  <div class="sea">
    <div class="mermaid"><div class="fishing"></div></div>                
    <div class="fish"></div>
  </div>
</div>

The fishing class is inside the mermaid class for demo purposes only.

And here we have some CSS styles:

.sample1 .sea, .sample1 .mermaid, .sample1 .fishing {
   height: 300px;
   width: 480px;
   position: relative;
}
      

     
.sample1 .sea {
   background: url(media/sea.png) repeat-x top left;          
}
     

      
.sample1 .mermaid {
   background: url(media/mermaid.svg) repeat-x bottom left;
}
     

  
.sample1 .fish {
  background: url(media/fish.svg) no-repeat;
  height: 70px;
  width: 100px;
  left: 30px;
  top: 90px;
  position: absolute;
}
     

      
.sample1 .fishing {
  background: url(media/fishing.svg) no-repeat top right 10px;
}

Result:

In this sample, I have three nested divs with background images and one more neighbor div, fish. You may imagine that the fish can be animated using JavaScript, or CSS3 transitions or animations.

Note, for the fishing class I’m using the new background positioning syntax, defined in CSS3. But for now it is supported only by IE9+ and Opera 11+; it does not yet work in Firefox 10 or Chrome 16. So the users of the last two cannot catch the fish. :(

Let’s continue. Is it possible to simplify this composition?

Multiple Backgrounds

This is when the multiple backgrounds come to the scene. This feature allows you to add more than one background at once, to the same element. Here is how it looks:

<div class="sample2">
  <div class="sea">                   
    <div class="fish"></div>
  </div>
</div>

And the styles:

.sample2 .sea {
  height: 300px;
  width: 480px;
  position: relative;
  background-image: url("media/fishing.svg"), url("media/mermaid.svg"), url("media/sea.png");  
  background-position: top right 10px, bottom left, top left;
  background-repeat: no-repeat, repeat-x, repeat-x ;
}



.sample2 .fish {
  background: url("media/fish.svg") no-repeat;
  height: 70px;
  width: 100px;
  left: 30px;
  top: 90px;
  position: absolute;
}

To define multiple backgrounds you should use the background-image rule by enumerating your images (comma-separated). You may also use other rules to set a position, repeating mode, and other attributes to each of the images—just write them also using a comma-separated list for the corresponding rule. Note the order of images: they are listed left to right starting with the uppermost one and ending with the lowest one.

The result is 100% identical:

In One Rule

If you don’t need your fish to swim in an independent block, the whole background can be written in one simple rule:

<div class="sample3">
  <div class="sea"></div>
</div>

Styles:

.sample3 .sea {
  height: 300px;
  width: 480px;
  position: relative;
  background-image:  url("media/fishing.svg"), url("media/mermaid.svg"), url("media/fish.svg"), url("media/sea.png");  
  background-position: top right 10px, bottom left, 30px 90px, top left;
  background-repeat: no-repeat, repeat-x;
}

I’m not showing the same picture one more time, but trust me—it is equal to the two images above. Look at the styles one more time, especially on the background-repeat rule. According to the spec, if a part of the list is omitted, the UA (browser) should repeat the present list to fill the rest.

In our case, it is equal to the following definition:

background-repeat: no-repeat, repeat-x, no-repeat, repeat-x;

Shorter Version

If you remember, in CSS2.1 it is possible to describe a background image in a one short background rule. What about multiple backgrounds? Actually, you can also use the background rule for multiple backgrounds:

.sample4 .sea {
  height: 300px;
  width: 480px;
  position: relative;
  background: url("media/fishing.svg") top right 10px no-repeat,
              url("media/mermaid.svg") bottom left repeat-x,
              url("media/fish.svg") 30px 90px no-repeat,
              url("media/sea.png") repeat-x;                
}

But note that you can’t easily omit arguments unless the values are equal to the default ones. Also, if you would like to define the color of the background you should do it in the last layer.

Dynamic Images

Here is what we already know: if your background is mostly static—it may depend on the container size (i.e. if you are using % length so that some layers will shift on resizing window)—then the magic of multiple backgrounds is useful as it really simplifies the page structure. But what if you need to animate some of the layers using JavaScript (move, rotate and so on)?

I have a real life sample—the dandelion theme on the Yandex website (Russian search provider, YNDX):

If you look into the source code (press F12 in your IE to open devtools) you will find code like this:

<div class=b-skin-bg sizcache="272" sizset="0">
  ​<div class=b-fluff-bg sizcache="272" sizset="0">
​​    <div class=b-fluff__sky sizcache="272" sizset="0">
      ​​​<div style="background-position: 3244px 0px" class=b-fluff__cloud></div>
        ​​​<div style="width: 1200px" class=b-max-width sizcache="214" sizset="0">
          <div class=b-fluff__placeholder sizcache="302" sizset="0">
          <div style="bottom: 105px; display: none; left: 940px" class="b-fluff__item b-fluff_item_3" jQuery1328289994769="30"></div>
          ​​​​​<div style="bottom: 50px; display: none; left: 879px" class="b-fluff__item b-fluff_item_3" jQuery1328289994769="31"></div>
          ​​​​​<div style="bottom: 105px; display: none; left: 940px" class="b-fluff__item b-fluff_item_3" jQuery1328289994769="32"></div>
…
        ​​​​</div>
      </div>
    </div>
  </div>
</div>

The divs with classes b-fluff-bg, b-fluff__cloud & b-fluff__item have the CSS rules applied, adding overlaying background images. The background with cloud scroll from left to right, and the backgrounds with dandelion seeds are flying across the screen.

Is it possible to rewrite such a composition using CSS3 multiple backgrounds? Actually, yes, but only if 1) it is supported in all target browsers and 2) continue reading ;)

How can we make our multiple backgrounds more dynamic? Internally, the browser parses every background rule into separate background-* rules for each of the attributes. It is very useful if you need to change only one of the attributes. For example, you can use the background-position rule to shift your images. But there are some penalties for dealing with multiple backgrounds: if you are going to move only one layer, you still need to rewrite this rule for all layers.

To animate our sea background, we can use the following JavaScript code:

$(document).ready(function() {
  var sea = $(".sample5 .sea")[0];
  var fishesX = 30;
  var fishesY = 90;
  var fishX = 0;
  var fishY = 0;
  var mermaidX = 0;
  var t = 0;
 
  function animationLoop() {
      fishesY = 90 + Math.floor(30 * Math.sin(t++ / 180.0));
      if(--fishesX  480) mermaidX = 0;
      fishY = -10 + (10 * Math.cos(t * 0.091));
      fishX = 10 + (5 * Math.sin(t * 0.07));
 
      sea.style.backgroundPosition = "top " + fishY + "px right " + fishX + "px, " + mermaidX + "px bottom," + fishesX + "px " + fishesY + "px, top left";
 
      window.requestAnimFrame(animationLoop);
  }
  animationLoop();
});

Where:

window.requestAnimFrame = (function() {
  return
      window.requestAnimationFrame ||
      window.msRequestAnimationFrame ||
      window.mozRequestAnimationFrame ||
      window.oRequestAnimationFrame ||
      window.webkitRequestAnimationFrame ||
      (function(callback) { window.setTimeout(callback, 1000 / 60); });
})();

Result (video):

You may also use CSS3 Transitions or Animations, but that is a good topic for a separate discussion.

Parallax and Interactivity

Finally, using similar techniques you can easily add some parallax effects or other interactive effects to your background:

Multiple backgrounds are useful in such scenarios, and while we are talking only about backgrounds (not the content), using them is definitely a good way to avoid polluting the HTML code with complex, unnecessary elements. But as I said, there are some penalties if you need to build a complex and dynamic background: you cannot access a separate layer by ID, class or any other parameter. You should remember the order of layers in your code, and to change an attribute for just one layer you will need to build a string describing this attribute for all the layers you have. To update one layer, you need to update the whole composition:

sea.style.backgroundPosition = "top " + fishY + "px right " + fishX + "px, " + mermaidX + "px bottom," + fishesX + "px " + fishesY + "px, top left";

I’m sure it is possible to build a nice and useful JavaScript library which will virtualize all these layers and provide an easy way to change attributes for a separate layer, keeping clean your HTML code and the DOM.

Compatibility

All modern browsers including IE10 and 9 support multiple backgrounds. You may also use some tools like Modernizr to provide some level of compatibility for older browsers, i.e. by providing an alternative background. As Chris Coyier wrote in his article on the stacking order of multiple backgrounds, you can use the following approach:

.multiplebgs body {
  /* Awesome multiple BG declarations that transcend reality and impress chicks */
}



.no-multiplebgs body {
  /* laaaaaame fallback */
}

If you are confused about using JavaScript to provide backward-compatibility for the new CSS3 rules, you can just define the background property twice (but this approach may result in unnecessary downloads for modern browsers, depending on how they process such rules):

/* multiple bg fallback */
background: #000 url(...) ...;
/* Awesome multiple BG declarations that transcend reality and impress chicks */
background: url(...), url(...), url(...), #000 url(...);

And finally, if you wish to know it: yes, you can use multiple backgrounds in you Windows 8 metro style apps built with HTML and JavaScript.

PS Check this phenomenal article about the Cicada Principle by Alex Walker.

Note
CSS properties discussed in this article are defined in the CSS3 Backgrounds and Borders module, which is currently in the Working Draft status. Meanwhile it seems to be quite stable it still can change in details.

CSS Master, 3rd Edition