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Create a Masked Background Effect With CSS

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What You'll Be Creating

Today we're going to be stepping through a really cool technique you can use to create an effect that's a little bit like parallax scrolling, yet doesn't need any JavaScript; it can be achieved very simply through pure CSS.

Start by checking out the live demo to see what you're going to learn (you'll need to view on a desktop/laptop to see the effect).

This technique could be used to create great product description pages, or even something akin to a Powerpoint/Keynote presentation, and would be a great potential fit for online story illustration.

Here's how you do it.

It's All in the CSS

The key to this technique the CSS background-attachment: fixed;, available to us since CSS 2.1. Any background image with this styling applied to it will remain in a fixed position relative to the window (not the element it's applied to). We'll use it to keep our illustrations in place while our content scrolls independently alongside it.

A couple of things to be aware of with this CSS property are that as background images will be fixed relative to the window, their position won't be effected by things like margins in the way a regular background image would.

You should also know that while the property works wonderfully in all desktop browsers, it doesn't currently work on Chrome mobile and can be a little jerky on other mobile browsers. So, while your visitors will still see your images just fine, the scrolling effect itself is best viewed on desktop platforms.

How It's Done

The basic steps to achieve what you see in the demo are:

  1. Create a container element and add your content to it.
  2. Set the container (a div in our case) to have padding on one side of around 50% width, therefore pushing the content over to the other side.
  3. Add a background image, also at around 50% width, and position it on the opposite side to the content.
  4. Set background-attachment: fixed; and watch the scrolling magic!

Let's go through how this all happens step by step. You'll want to grab the source files for this tutorial so you have the required images.

1. Basic Setup

Start by creating a project folder and adding an index.html file to it, as well as a css folder with a file named style.css added to it. Copy and paste the four images from the source file zip you downloaded into a folder named images.

Add this HTML to index.html:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>A Visual Demonstration of background-attachment: fixed;</title>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link href="css/style.css" rel="stylesheet" type="text/css">
    <link href="//fonts.googleapis.com/css?family=Alike|Roboto:900" rel="stylesheet" type="text/css">
  </head>
  <body>
    <div class="content right illustration_01">
        <h2>Scroll Down and Watch What Happens</h2>
        <p>Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do: once or twice she had peeped into the book her sister was reading, but it had no pictures or conversations in it, `and what is the use of a book,' thought Alice `without pictures or conversation?'</p>
        <p>So she was considering in her own mind (as well as she could, for the hot day made her feel very sleepy and stupid), whether the pleasure of making a daisy- chain would be worth the trouble of getting up and picking the daisies, when suddenly a White Rabbit with pink eyes ran close by her.</p>
        <p>There was nothing so very remarkable in that; nor did Alice think it so very much out of the way to hear the Rabbit say to itself, `Oh dear! Oh dear! I shall be late!' (when she thought it over afterwards, it occurred to her that she ought to have wondered at this, but at the time it all seemed quite natural); but when the Rabbit actually took a watch out of its waistcoat- pocket, and looked at it, and then hurried on, Alice started to her feet, for it flashed across her mind that she had never before seen a rabbit with either a waistcoat-pocket, or a watch to take out of it, and burning with curiosity, she ran across the field after it, and fortunately was just in time to see it pop down a large rabbit-hole under the hedge.</p>
        <p>In another moment down went Alice after it, never once considering how in the world she was to get out again.</p>
        <p>The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well.</p>
        <p>Either the well was very deep, or she fell very slowly, for she had plenty of time as she went down to look about her and to wonder what was going to happen next. First, she tried to look down and make out what she was coming to, but it was too dark to see anything; then she looked at the sides of the well, and noticed that they were filled with cupboards and book-shelves; here and there she saw maps and pictures hung upon pegs. She took down a jar from one of the shelves as she passed; it was labelled `ORANGE MARMALADE', but to her great disappointment it was empty: she did not like to drop the jar for fear of killing somebody, so managed to put it into one of the cupboards as she fell past it.</p>
        <p>`Well!' thought Alice to herself, `after such a fall as this, I shall think nothing of tumbling down stairs! How brave they'll all think me at home! Why, I wouldn't say anything about it, even if I fell off the top of the house!' (Which was very likely true.) </p>
    </div>  
  </body>
</html>

What we're doing here is setting up our basic HTML shell, loading in our stylesheet and some Google Fonts, then creating our first div content container that we'll be applying this technique to.

The div container has three classes:

  1. .content - used to set some properties that will be common to all content containers.
  2. .right - identifies that this container should have right-aligned content (we'll also be working with a left-aligned container later)
  3. .illustration_01 - used to set the specific background image and color for this container

Styling

Now we're ready for some CSS. Start by adding some basic normalizing and formatting code to your style.css file:

* {
  box-sizing: border-box;
}
html {
  font-size: 1em;
  font-family: 'Alike';
  background-color: #262626;
  color: #d9d9d9;
}
body {
  margin: 0;
}
img {
  max-width: 100%;
  height: auto;
}
h1,h2,h3,h4,h5,h6 {
  font-family: 'Roboto';
  line-height: 1.313em;
}
h1 {
  font-size: 3em;
  margin: 0.563em 0;
}
h2 {
  font-size: 2.25em;
  margin: 0.625em 0;
}
h3 {
  font-size: 1.5em;
  margin: 1.313em 0;
}
h4 {
  font-size: 1.313em;
  margin: 1.313em 0;
}
h5 {
  font-size: 1.125em;
  margin: 1.313em 0;
}
h6 {
  font-size: 1em;
  margin: 0.75em 0;
}

Now for the .content class. Add this to the bottom of your stylesheet:

.content {
  font-size: 1.875rem;
  color: #262626;
  background-size: 49% auto;
  background-attachment: fixed;
  background-repeat: no-repeat;
}

This class is where most of the magic happens. For the text, we set our font size and color. For the background you'll see we start by setting background-size to 49% auto. 

This means that the background image will always stretch or shrink to fill 49% of the page width, and the height will adjust proportionately. We are using a value of 49% instead of 50% because otherwise Firefox shows a weird line artefact in the middle of the screen.

We then set background-fixed which, as you already know from the description above, makes the background image stay put when we scroll, and makes its positioning relative to the window rather than the container it's applied to.

Finally we set background-repeat: no-repeat; to ensure our image appears just once on the page.

Next add the .right class to your stylesheet:

.right {
  padding: 1.618em 6.472em 3.236em 50%;
  background-position: 0 50%;
}

This last class places the text content on one half of the screen and the background image on the other.

The background-position setting tells the background image to position itself zero pixels from the left side of the window and to align itself halfway down from the top of the window.

Lastly, add the .illustration_01 class:

.illustration_01 {
  background-color: #00c17b;
  background-image: url("../images/minipadwhite.png");
}

This sets the specific background image and color we want for this content container.

Check out your site and you should now see this:

When you scroll down you should see the content slide along while the image stays exactly where it is.

2. Add a Second Container

Let's add another content container, this one with content aligned to the left.

Add this content container HTML below your last div:

    <div class="content left illustration_02">
      <h2>Fixed Background Scrolling Effect</h2>
      <p>Down, down, down. Would the fall never come to an end! `I wonder how many miles I've fallen by this time?' she said aloud. `I must be getting somewhere near the centre of the earth. Let me see: that would be four thousand miles down , I think--' (for, you see, Alice had learnt several things of this sort in her lessons in the schoolroom, and though this was not a very good opportunity for showing off her knowledge, as there was no one to listen to her, still it was good practice to say it over) `--yes, that's about the right distance--but then I wonder what Latitude or Longitude I've got to?' (Alice had no idea what Latitude was, or Longitude either, but thought they were nice grand words to say .)</p>
      <p>Presently she began again. `I wonder if I shall fall right through the earth! How funny it'll seem to come out among the people that walk with their heads downward! The Antipathies, I think--' (she was rather glad there was no one listening, this time, as it didn't sound at all the right word) `--but I shall have to ask them what the name of the country is, you know. Please, Ma' am, is this New Zealand or Australia?' (and she tried to curtsey as she spoke-- fancy curtseying as you're falling through the air! Do you think you could manage it?) `And what an ignorant little girl she'll think me for asking! No, it'll never do to ask: perhaps I shall see it written up somewhere.'</p>
      <p>Down, down, down. There was nothing else to do, so Alice soon began talking again. `Dinah'll miss me very much to-night, I should think!' (Dinah was the cat .) `I hope they'll remember her saucer of milk at tea-time. Dinah my dear! I wish you were down here with me! There are no mice in the air, I'm afraid, but you might catch a bat, and that's very like a mouse, you know. But do cats eat bats, I wonder?' And here Alice began to get rather sleepy, and went on saying to herself, in a dreamy sort of way, `Do cats eat bats? Do cats eat bats?' and sometimes, `Do bats eat cats?' for, you see, as she couldn't answer either question, it didn't much matter which way she put it. She felt that she was dozing off, and had just begun to dream that she was walking hand in hand with Dinah, and saying to her very earnestly, `Now, Dinah, tell me the truth: did you ever eat a bat?' when suddenly, thump! thump! down she came upon a heap of sticks and dry leaves, and the fall was over.</p>
      <p>Alice was not a bit hurt, and she jumped up on to her feet in a moment: she looked up, but it was all dark overhead; before her was another long passage, and the White Rabbit was still in sight, hurrying down it. There was not a moment to be lost: away went Alice like the wind, and was just in time to hear it say, as it turned a corner, `Oh my ears and whiskers, how late it's getting!' She was close behind it when she turned the corner, but the Rabbit was no longer to be seen: she found herself in a long, low hall, which was lit up by a row of lamps hanging from the roof.</p>
      <p>There were doors all round the hall, but they were all locked; and when Alice had been all the way down one side and up the other, trying every door, she walked sadly down the middle, wondering how she was ever to get out again. </p>
    </div>

Notice this time we use the class .left instead of .right and we've upped the illustration number so the class .illustration_01 is replaced with .illustration_02

Add the following two new classes to your stylesheet:

.left {
  padding: 1.618em 50% 3.236em 6.472em;
  background-position: 100% 50%;
}
.illustration_02 {
  background-color: #e8697b;
  background-image: url("../images/minipadblack.png");
}

This time we have the 50% padding applied to the right side of the container so the content will be pushed to the left, and the background is horizontally positioned at 100%, i.e all the way to the right. We also add a different color and image to this container's background.

Check out your site again and start scrolling down. When you reach the end of the first container you should see the second one start coming up, scrubbing over the top of your first image and gradually revealing your second.

3. Insert a Separator

It enhances the effect of this technique if there is a separator between the two containers so let's add that now.

In between your two container divs add this HTML:

    <section class="separator">
      <h3>Another Section Starts Here</h3>
    </section>

And add the .separator class to your stylesheet:

.separator {
  font-size: 1.875rem;
  padding: 1.618em 0;
  text-align: center;
}

When you refresh your site you should now have a nice separator between your containers:

4. Third and Fourth Containers

You can now enter the code for your remaining separators and content containers.

Add this HTML below your existing divs:

    <section class="separator">
      <h3>Another Section Starts Here</h3>
    </section>
    <div class="content right illustration_03">
      <h2>Great For Product Presentations</h2>
      <p>Suddenly she came upon a little three-legged table, all made of solid glass; there was nothing on it except a tiny golden key, and Alice's first thought was that it might belong to one of the doors of the hall; but, alas! either the locks were too large, or the key was too small, but at any rate it would not open any of them. However, on the second time round, she came upon a low curtain she had not noticed before, and behind it was a little door about fifteen inches high: she tried the little golden key in the lock, and to her great delight it fitted!</p>
      <p>Alice opened the door and found that it led into a small passage, not much larger than a rat-hole: she knelt down and looked along the passage into the loveliest garden you ever saw. How she longed to get out of that dark hall, and wander about among those beds of bright flowers and those cool fountains, but she could not even get her head though the doorway; `and even if my head would go through,' thought poor Alice, `it would be of very little use without my shoulders. Oh, how I wish I could shut up like a telescope! I think I could, if I only know how to begin.' For, you see, so many out-of-the-way things had happened lately, that Alice had begun to think that very few things indeed were really impossible.</p>
      <p>There seemed to be no use in waiting by the little door, so she went back to the table, half hoping she might find another key on it, or at any rate a book of rules for shutting people up like telescopes: this time she found a little bottle on it, (`which certainly was not here before,' said Alice,) and round the neck of the bottle was a paper label, with the words `DRINK ME' beautifully printed on it in large letters.</p>
      <p>It was all very well to say `Drink me,' but the wise little Alice was not going to do that in a hurry. `No, I'll look first,' she said, `and see whether it's marked "poison" or not'; for she had read several nice little histories about children who had got burnt, and eaten up by wild beasts and other unpleasant things, all because they would not remember the simple rules their friends had taught them: such as, that a red-hot poker will burn you if you hold it too long; and that if you cut your finger very deeply with a knife, it usually bleeds; and she had never forgotten that, if you drink much from a bottle marked `poison,' it is almost certain to disagree with you, sooner or later.</p>
      <p>However, this bottle was NOT marked `poison,' so Alice ventured to taste it, and finding it very nice, (it had, in fact, a sort of mixed flavour of cherry- tart, custard, pine-apple, roast turkey, toffee, and hot buttered toast,) she very soon finished it off. </p>
    </div>
    <section class="separator">
      <h3>Another Section Starts Here</h3>
    </section>
    <div class="content left illustration_04">
      <h2>Simple Technique Using Pure CSS</h2>
      <p> `What a curious feeling!' said Alice; `I must be shutting up like a telescope .'</p>
      <p>And so it was indeed: she was now only ten inches high, and her face brightened up at the thought that she was now the right size for going though the little door into that lovely garden. First, however, she waited for a few minutes to see if she was going to shrink any further: she felt a little nervous about this; `for it might end, you know,' said Alice to herself, `in my going out altogether, like a candle. I wonder what I should be like then?' And she tried to fancy what the flame of a candle is like after the candle is blown out, for she could not remember ever having seen such a thing.</p>
      <p>After a while, finding that nothing more happened, she decided on going into the garden at once; but, alas for poor Alice! when she got to the door, she found he had forgotten the little golden key, and when she went back to the table for it, she found she could not possibly reach it: she could see it quite plainly through the glass, and she tried her best to climb up one of the legs of the table, but it was too slippery; and when she had tired herself out with trying, the poor little thing sat down and cried.</p>`Come, there's no use in crying like that!' said Alice to herself, rather sharply; `I advise you to leave off this minute!' She generally gave herself very good advice, (though she very seldom followed it), and sometimes she scolded herself so severely as to bring tears into her eyes; and once she remembered trying to box her own ears for having cheated herself in a game of croquet she was playing against herself, for this curious child was very fond of pretending to be two people. `But it's no use now,' thought poor Alice, `to pretend to be two people! Why, there's hardly enough of me left to make ONE respectable person!'
      <p>Soon her eye fell on a little glass box that was lying under the table: she opened it, and found in it a very small cake, on which the words `EAT ME' were beautifully marked in currants. `Well, I'll eat it,' said Alice, `and if it makes me grow larger, I can reach the key; and if it makes me grow smaller, I can creep under the door; so either way I'll get into the garden, and I don't care which happens!'</p>
      <p>She ate a little bit, and said anxiously to herself, `Which way? Which way?', holding her hand on the top of her head to feel which way it was growing, and she was quite surprised to find that she remained the same size: to be sure, this generally happens when one eats cake, but Alice had got so much into the way of expecting nothing but out-of-the-way things to happen, that it seemed quite dull and stupid for life to go on in the common way.</p>
      <p>So she set to work, and very soon finished off the cake. </p>
    </div>
    <section class="separator">
      <h1>THE END</h1>
    </section>

And this CSS to your stylesheet:

.illustration_03 {
  background-color: #14b29a;
  background-image: url("../images/miniwhite.png");
}
.illustration_04 {
  background-color: #80b9f1;
  background-image: url("../images/miniblack.png");
}

You should now have your entire display in place with a third and fourth content container showing:

As well as a final separator to cap it off:

5. Make it Responsive

The very last thing you'll need to do is account for varying screen sizes. When the viewport gets too small to comfortably accommodate our background images we want to switch them to inline images instead.

At the top of each of your content containers, inside the opening divs and above the text, add a figure with the class .smallscreen and inside that place an img tag to load each of the images currently used in the backgrounds:

First content container:

      <figure class="smallscreen">
        <image src="images/minipadwhite.png">
      </figure>

Second content container:

      <figure class="smallscreen">
        <image src="images/minipadblack.png">
      </figure>

Third content container:

      <figure class="smallscreen">
        <image src="images/miniwhite.png">
      </figure>

Fourth content container:

      <figure class="smallscreen">
        <image src="images/miniblack.png">
      </figure>

We're going to use the .smallscreen class to hide this inline image by default, but to show it when we get to a smaller screen size.

Add the following class to your stylesheet:

.smallscreen {
  display: none;
}

We'll now add the media queries that will handle whether the background or inline images are displayed. They will also progressively scale down the size of the text and spacing in the layout so we fit things in nicely at all viewport widths.

Add these media queries to your stylesheet:

@media (max-width: 106.25rem) {
  .wrapper,
  .separator {
    font-size: 1.6875rem;
  }
}
@media (max-width: 93.75rem) {
  .content,
  .separator {
    font-size: 1.5rem;
  }
  .right {
    padding: 1.618em 4.854em 1.618em 50%;
  }
  .left {
    padding: 1.618em 50% 1.618em 4.854em;
  }
}
@media (max-width: 81.25rem) {
  .content,
  .separator {
    font-size: 1.3125rem;
  }
  .right {
    padding: 1.618em 3.236em 1.618em 45%;
    background-size: 44% auto;
    background-position: 0 55%;
  }
  .left {
    padding: 1.618em 45% 1.618em 3.236em;
    background-size: 44% auto;
    background-position: 100% 55%;
  }
}
@media (max-width: 68.75rem) {
  .content,
  .separator {
    font-size: 1.125rem;
  }
  .right {
    padding: 1.618em 3.236em 1.618em 40%;
    background-size: 39% auto;
    background-position: 0 60%;
  }
  .left {
    padding: 1.618em 40% 1.618em 3.236em;
    background-size: 39% auto;
    background-position: 100% 60%;
  }
}
@media (max-width: 50rem) {
  .smallscreen {
    display: block;
  }
  .right {
    padding: 1.618em 3.236em;
    background-image: none;
  }
  .left {
    padding: 1.618em 3.236em;
    background-image: none;
  }
}
@media (max-width: 31.25rem) {
  .right {
    padding: 1.618em 1.618em;
  }
  .left {
    padding: 1.618em 1.618em;
  }
}
@media (max-width: 12rem) {
  html {
    min-width: 12rem;
  }
}

The first four media queries are simply reducing the text font size and padding inside the content containers progressively to suit the available screen width.

In the fifth media query of max-width: 50rem we include code that makes the .smallscreen class visible, removes our 50% side padding from the content containers, and hides the background images. When this media query kicks in we'll no longer see the large fixed background images, and instead we'll see regular images at the top of each content container.

Now when you refresh your site you should see it smoothly scale down along with all viewport widths, until you see this at its smallest size:

Conclusion

Even after so many years working with CSS I never fail to be surprised by the ever increasing number of awesome things you can do with it. And the simpler the technique is, the more impressive.

Try out this little gem, it's so quick and easy you're likely to get hooked!

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