IEEE Computer Society

IEEE Computer Society

IEEE Computer Society

IEEE Computer Society

IEEE Computer Society

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE / Institute of Electrical and Electronics Engineers Incorporated

IEEE Computer Society

IEEE Computer Society

Online Resource

Online Resource

Online Resource

Online Resource

Callow, Jon, 1964- author

Katsipis, Maroulia, author

Online Resource

Rotch Library - BR127.J49 2019

Rotch Library - BP75.G78 2018

Hayden Library - QP495.M37 2017

Rousselle, Duane, author

Two of the world’s most famous research chimpanzees have finally retired. Hercules and Leo arrived at a chimp sanctuary in Georgia last week. Sarah Crespi checks in with Online News Editor David Grimm on the increasing momentum for research chimp retirement since the primates were labeled endangered species in 2015. Sarah also interviews freelancer Sophia Chen about her piece on x-ray ghost imaging—a technique that may lead to safer medical imaging done with cheap, single-pixel cameras. David Malakoff joins Sarah to talk about the big boost in U.S. science funding signed into law over the weekend. Finally, Jen Golbeck interviews author Stephanie Elizabeth Mohr on her book First in Fly: Drosophila Research and Biological Discovery for our monthly books segment. This week’s episode was edited by Podigy. Listen to previous podcasts. [Image: Crystal Alba/Project Chimps; Music: Jeffrey Cook]

Imagine having a rat clinging to your back, sucking out your fat stores. That’s similar to what infested bees endure when the Varroa destructor mite comes calling. Some bees fight back, wiggling, scratching, and biting until the mites depart for friendlier backs. Now, researchers, professional beekeepers, and hobbyists are working on ways to breed into bees these mite-defeating behaviors to rid them of these damaging pests. Host Sarah Crespi and Staff Writer Erik Stokstad discuss the tactics of, and the hurdles to, pesticide-free mite control. Also this week, Sarah talks to Philip Kragel of the Institute of Cognitive Science at the University of Colorado in Boulder about training an artificial intelligence on emotionally charged images. The ultimate aim of this research: to understand how the human visual system is involved in processing emotion. And in books, Kate Eichorn, author of The End of Forgetting: Growing Up with Social Media, joins books host Kiki Sanford to talk about how the monetization of digital information has led to the ease of social media sharing and posting for kids and adults. This week’s episode was edited by Podigy. Download a transcript (PDF)  Listen to previous podcasts. About the Science Podcast [Image: Steve Baker/Flickr; Music: Jeffrey Cook]

Bilan, Stepan, 1962- author

Location Media Resources Collection

Call No. HQ1665.45 .J4 2015

Eric Meyer gift wraps the most awkwardly shaped of boxes using nothing but CSS, HTML and a little curl of ribbon. No matter how well you plan and how much paper you have at your disposal, sometimes you just need to slant the gift to the side.

We have a lot of new layout tools at our disposal these days—flexbox is finally stable and interoperable, and Grid very much the same, with both technologies having well over 90% support coverage. In that light, we might think there’s no place for old tricks like negative margins, but I recently discovered otherwise.

Over at An Event Apart, we’ve been updating some of our landing pages, and our designer thought it would be interesting to have slanted images of speakers at the tops of pages. The end result looks like this.

The interesting part is the images. I wanted to set up a structure like the following, so that it will be easy to change speakers from time to time while preserving accessible content structures:

<div id="page-top">
  <ul class="monoliths">
    <li>
      <a href="https://aneventapart.com/speakers/rachel-andrew"> 
        <img src="/img/rachel-andrew.jpg" alt=""> 
        <div> 
          <strong>Rachel Andrew</strong> CSS Grid 
        </div> 
      </a>
    </li>
    <li>
      <a href="https://aneventapart.com/speakers/derek-featherstone"> 
        <img src="/img/derek-featherstone.jpg" alt=""> 
        <div> 
          <strong>Derek Featherstone</strong> Accessibility 
        </div> 
      </a>
    </li>
    <li>
      …
    </li>
    <li>
      …
    </li>
  </ul>
</div>

The id value for the div is straightforward enough, and I called the ul element monoliths because it reminded me of the memorial monoliths at the entrance to EPCOT in Florida. I’m also taking advantage of the now-ubiquitous ability to wrap multiple elements, including block elements, in a hyperlink. That way I can shove the image and text structures in there, and make the entire image and text below it one link.

Structure is easy, though. Can we make that layout fully responsive? I wondered. Yes we can. Here’s the target layout, stripped of the navbar and promo copy.

So let’s start from the beginning. The div gets some color and text styling, and the monoliths list is set to flex. The images are in a single line, after all, and I want them to be flexible for responsive reasons, so flexbox is 100% the right tool for this particular job.

#page-top { 
  background: #000; 
  color: #FFF; 
  line-height: 1; 
} 
#page-top .monoliths { 
  display: flex; 
  padding-bottom: 1em; 
  overflow: hidden; 
}

I also figured, let’s give the images a simple basis for sizing, and set up the hyperlink while we’re at it.

#page-top .monoliths li { 
  width: 25%; 
} 
#page-top .monoliths a { 
  color: inherit; 
  text-decoration: inherit; 
  display: block; 
  padding: 1px; 
}

So now the list items are 25% wide—I can say that because I know there will be four of them—and the links pick up the foreground color from their parent element. They’re also set to generate a block box.

At this point, I could concentrate on the images. They need to be as wide as their parent element, but no wider, and also match height. While I was at it, I figured I’d create a little bit of space above and below the captioning text, and make the strong elements containing speakers’ names generate a block box.

#page-top .monoliths img { 
  display: block; 
  height: 33rem; 
  width: 100%; 
} 
#page-top .monoliths div { 
  padding: 0.5em 0; 
} 
#page-top .monoliths strong { 
  display: block; 
  font-weight: 900; 
}

It looks like the speakers were all cast into the Phantom Zone or something, so that needs to be fixed. I can’t physically crop the images to be the “correct” size, because there is no correct size: this needs to work across all screen widths. So rather than try to swap carefully-sized images in and out at various breakpoints, or complicate the structure with a wrapper element set to suppress overflow of resized images, I turned to object-fit.

#page-top .monoliths img { 
  display: block; 
  height: 33rem; 
  width: 100%; 
  object-fit: cover; 
  object-position: 50% 20%; 
}

If you’ve never used object-fit, it’s a bit like background-size. You can use it to resize image content within the image’s element box without creating distortions. Here, I set the fit sizing to cover, which means all of the img element’s element box will be covered by image content. In this case, it’s like zooming in on the image content. I also set a zooming origin with object-position, figuring that 50% across and 20% down would be in the vicinity of a speaker’s face, given the way pictures of people are usually taken.

This is fairly presentable as-is—a little basic, perhaps, but it would be fine to layer the navbar and promo copy back over it with Grid or whatever, and call it a day. But it’s too square and boxy. We must go further!

To make that happen, I’m going to take out the third and fourth images temporarily, so we can see more clearly how the next part works. That will leave us with Rachel and Derek.

The idea here is to clip the images to be slanted, and then pull them close to each other so they have just a little space between them. The first part is managed with clip-path, but we don’t want to pull the images together unless their shapes are being clipped. So we set up a feature query.

@supports (clip-path: polygon(0 0)) or (-webkit-clip-path: polygon(0 0)) { 
  #page-top .monoliths li { 
    width: 37.5%; 
  } 
}

I decided to test for both the un-prefixed and WebKit-prefixed versions of clip-path because Safari still requires the prefix, and I couldn’t think of a good reason to penalize Safari’s users for the slowness of its standards advancement. Then I made the images wider, taking them from 25% to 37.5%, which makes them half again as wide.

Thanks to object fitting, the images don’t distort when I change their parent’s width; they just get wider and scale up the contents to fit. And now, it is time for clipping!

@supports (clip-path: polygon(0 0)) or (-webkit-clip-path: polygon(0 0)) { 
  #page-top .monoliths li { 
    width: 37.5%; 
    -webkit-clip-path: polygon(25% 0, 100% 0, 75% 100%, 0 100%); 
    clip-path: polygon(25% 0, 100% 0, 75% 100%, 0 100%); 
  } 
}

Each coordinate pair in the polygon() is like the position pairs in background-position or object-position: the horizontal distance first, followed by the vertical distance. So the first point in the polygon is 25% 0, which is 25% of the way across the element box, and no distance down, so right at the top edge. 100% 0 is the top right corner. 75% 100% is on the bottom edge, three-quarters of the way across the element, and 0 100% is the bottom left corner. That creates a polygon that’s a strip three-quarters the full width of the element box, and runs from bottom left to top right.

Now we just have to pull them together, and this is where old tricks come back into play: all we need is a negative right margin to bring them closer together.

#page-top .monoliths li { 
  width: 37.5%; 
  margin-right: -7.5%; 
  -webkit-clip-path: polygon(25% 0, 100% 0, 75% 100%, 0 100%); 
  clip-path: polygon(25% 0, 100% 0, 75% 100%, 0 100%); 
}

The separation between them is a little wider than we were originally aiming for, but let’s see what happens when we add the other two images back in and let flexbox do its resizing magic.

Notice how the slants actually change shape as the screen gets narrower or wider. This is because they’re still three-quarters the width of the image element’s box, but the width of that box is changing as the screen width changes. That means at narrow widths, the slant is much steeper, whereas at wide widths, the slant is more shallow. But since the clipping path’s coordinates were all set with percentage distances, they all stay parallel to each other while being completely responsive to changes in screen size. An absolute measure like pixels would have failed.

But how did the images get closer together just by adding in two more? Because the list items’ basic sizing added up to more than 100%, and they’re all set to flex-shrink: 1. No, you didn’t miss a line in the CSS: 1 is the default value for flex-shrink. Flex items will shrink by default, which after all is what we should expect from a flexible element. If you want to know how much they shrunk, and why, here’s what Firefox’s flex inspector reports.

When there were only two list items, there was space enough for both to be at their base size, with no shrinkage. Once we went to four list items, there wasn’t enough space, so they all shrank down. At that point, having a negative right margin of -7.5% was just right to pull them together to act as a unit.

So, now they’re all nicely nestled together, and fully responsive! The captions need a little work, though. Notice how they’re clipped off a bit on the left edge, and can be very much clipped off on the right side at narrower screen widths? This happens because the li elements are being clipped, and that clipping applies to all their contents, images and text alike. And we can’t use overflow to alter this: clipped is clipped, not overflowed.

Fortunately, all we really need to do is push the text over a small amount. Inside the feature query, I added:

#page-top .monoliths div { 
  padding-left: 2%;
  padding-right: 26%; 
}

This shifts the text just a bit rightward, enough to clear the clip path. On the right side, I padded the div boxes so their contents wouldn’t fall outside the clipped area and appear to slide under the next caption. We could also use margins here, but I didn’t for reasons I’ll make clear at the end.

At the last minute, I decided to make the text at least appear to follow the slants of the images. For that, I just needed to shift the first line over a bit, which I did with a bit more padding.

#page-top .monoliths strong { 
  padding-left: 1%; 
}

That’s all to the good, but you may have noticed the captions still overlap at really narrow screen widths. There are a lot of options here, from stacking the images atop one another to reverting to normal flow, but I decided to just hide the captions if things got too narrow. It reduces clutter without sacrificing too much in the way of content, and by leaving them still technically visible, they seem to remain accessible.

@media (max-width: 35rem) { 
  #page-top .monoliths div { 
    opacity: 0.01 
  } 
}

And that, as they say, is that! Fully responsive slanted images with text, in an accessible markup structure. I dig it.

I did fiddle around with the separations a bit, and found that a nice thin separator occurred around margin-right: -8%, whereas beefier ones could be found above -7%. And if you crank the negative margin value to something beyond -8%, you’ll make the images overlap entirely, no visible separation—which can be a useful effect in its own right.

I promised to say why I used padding for the caption text div rather than margins. Here’s why.

#page-top .monoliths div { 
  padding-left: 3%; 
  padding-right: 26%; 
  border-top: 2px solid transparent; 
  background: linear-gradient(100deg,hsl(292deg,50%,50%) 50%, transparent 85%); 
  background-clip: padding-box; 
}

It required a wee bit more padding on the left to look decent, and an alteration to the background clipping box in order to keep the purple from filling the transparent border area, but the end result is pretty nifty, if I do say so myself. Alternatively, we could drop the background gradient on the captions and put one in the background, with a result like this.

I have no doubt this technique could be extended, made more powerful, and generally improved upon. I really wished for subgrid support in Chrome, so that I could put everything on a grid without having to tear the markup structure apart, and there are doubtless even more interesting clipping paths and layout patterns to try out.

I hope these few ideas spark some much better ideas in you, and that you’ll share them with us!

About the author

Eric A. Meyer (@meyerweb) has been a burger flipper, a college webmaster, an early blogger, one of the original CSS Samurai, a member of the CSS Working Group, a consultant and trainer, and a Standards Evangelist for Netscape. Among other things, Eric co-wrote Design For Real Life with Sara Wachter-Boettcher for A Book Apart and CSS: The Definitive Guide with Estelle Weyl for O’Reilly, created the first official W3C test suite, assisted in the creation of microformats, and co-founded An Event Apart with Jeffrey Zeldman. Eric lives with his family in Cleveland, Ohio, which is a much nicer city than you’ve probably heard. He enjoys a good meal whenever he can and considers almost every form of music to be worthwhile.

More articles by Eric

A rapid animation of multiple layers of images for intros or page transitions.

Rapid Image Layers Animation was written by Mary Lou and published on Codrops.

Chapman, Elwood N

An accessible image replacement method using pseudo-elements and generated-content. This method works with images and/or CSS off, with semi-transparent images, doesn’t hide text from screen-readers or search engines, and provides fallback for IE 6 and IE 7.

Known support: Firefox 1.5+, Safari 3+, Chrome 3+, Opera 9+, IE 8+

What’s wrong with current methods?

The two most widely used image replacement techniques are the Gilder/Levin Method and the Phark Method. Both have different flaws.

The Gilder/Levin Method requires the addition of presentational HTML (an empty span) and doesn’t work with transparent images as the default text shows through. The Phark Method uses a negative text-indent to hide the text and so it is not visible when CSS is on and images are off.

Resurrecting the NIR method

Using pseudo-elements and generated-content as an image replacement technique isn’t a new idea. It was proposed and demonstrated by Paul Nash back in 2006. This is the Nash Image Replacement method.

<h1 class="nir">[content]</h1>

.nir {
   height: 100px; /* height of replacement image */
   padding: 0;
   margin: 0;
   overflow: hidden;
}

.nir:before {
   content: url(image.gif);
   display: block;
}

The height value is equal to that of the replacement image. Setting overflow:hidden ensures that the original content is not visible on screen when the image is loaded. The replacement image is inserted as generated content in the :before pseudo-element which is set to behave like a block element in order to push the element’s original content down.

What about IE 6 and IE 7?

Neither browser supports :before; if you need to support them you’ll have to rely on the Phark method. This can be done using conditional comments or safe IE6/7 hacks to serve alternative styles to legacy versions of IE .

<!--[if lte IE 7]>
<style>
.nir {
   height: 100px;
   padding: 0;
   margin: 0;
   overflow: hidden;
   text-indent: -9000px;
   background: url(image.gif) no-repeat 0 0;
}
</style>
<![endif]-->

Using the NIR method allows you to keep your HTML semantic and deliver improved accessibility to users of modern browsers. The Phark Method can then be served to IE 6 and IE 7.

Improving the NIR method

The first problem with NIR is that if images are disabled all browsers leave whitespace above the element’s content. Opera 10.5 even displays the text string “image”! If the height of the element is small enough this whitespace causes the element’s content to overflow and be partially or completely hidden when images are disabled.

Another consideration is what happens if an image doesn’t exist or fails to load. Safari and Chrome will display a “missing image” icon that cannot be removed. Once again, this can cause the element’s content to overflow and become partially or completely hidden to users.

A more robust version of the NIR method is the following modification:

.nir {
   height: 100px; /* height of replacement image */
   width: 400px; /* width of replacement image */
   padding: 0;
   margin: 0;
   overflow: hidden;
}

.nir:before {
   content: url(image.gif);
   display: inline-block;
   font-size: 0;
   line-height: 0;
}

Setting font-size and line-height to 0 avoids the whitespace problems in all browsers. Setting the element’s width equal to that of the replacement image and getting the pseudo-element to act as an inline-block helps minimise the problems in webkit browsers should an image fail to load.

Ideally browsers would avoid displaying anything in a pseudo-element when its generated-content image fails to load. If that were the case, the original NIR method would be all that is needed.

What about using sprites?

One of the most common uses of image replacement is for navigation. This often involves using a large sprite with :hover and :active states as a background image. It turns out that using sprites is not a problem for modern browsers. When using the modified-NIR method the sprite is included as a generated-content image that is positioned using negative margins.

This is an example that rebuilds the right-hand category navigation from Web Designer Wall using a sprite and the modified-NIR method.

<ul id="nav">
  <li id="nav-item-1"><a href="#non">Tutorials</a></li>
  <li id="nav-item-2"><a href="#non">Trends</a></li>
  <li id="nav-item-3"><a href="#non">General</a></li>
</ul>

/* modified-NIR */

#nav a {
  display: block;
  width: 225px;
  height: 46px;
  overflow: hidden;
}

#nav a:before {
   content:url(sprite.png);
   display:-moz-inline-box; /* for Firefox 1.5 & 2 */
   display:inline-block;
   font-size:0;
   line-height:0;
}

/* repositioning the sprite */

#nav-item-1 a:hover:before,
#nav-item-1 a:focus:before,
#nav-item-1 a:active:before {margin:-46px 0 0;}

#nav-item-2 a:before        {margin:-92px 0 0;}
#nav-item-2 a:hover:before,
#nav-item-2 a:focus:before,
#nav-item-2 a:active:before {margin:-138px 0 0;}

#nav-item-3 a:before        {margin:-184px 0 0;}
#nav-item-3 a:hover:before,
#nav-item-3 a:focus:before,
#nav-item-3 a:active:before {margin:-230px 0 0;}

/* :hover hack for IE8 if no a:hover styles declared */
#nav a:hover {cursor:pointer;}

For some reason IE8 refuses to reposition the image when the mouse is over the link unless a style is declared for a:hover. In most cases you will have declared a:hover styles for the basic links on your webpage, and this is enough. But it is worth being aware of this IE8 behaviour.

The addition of display:-moz-inline-box; is required to reposition the sprite in versions of Firefox prior to Firefox 3.0. They are very rare browsers but I’ve included it in case that level of legacy support is needed.

If you want image replacement in IE 6 and IE 7 the following additional styles can be served to those browsers using conditional comments.

/* Phark IR method */

#nav a {
   text-indent: -9000px;
   background: url(sprite.png) no-repeat;
}

/* repositioning the sprite */

#nav-item-1 a:hover,
#nav-item-1 a:active { background-position: 0 -46px; }

#nav-item-2 a        { background-position: 0 -92px; }
#nav-item-2 a:hover,
#nav-item-2 a:hover  { background-position: 0 -138px; }

#nav-item-3 a        { background-position: 0 -184px; }
#nav-item-3 a:hover,
#nav-item-3 a:active { background-position: 0 -230px; }

/* hack for IE6 */
#nav a:hover { margin: 0; }

The changes are fairly simple. But IE 6 applies the margins declared for a:hover:before to a:hover and so they need to be reset in the styles served to IE 6.

See the modified-NIR (using sprites) demo.

Drop-shadows are easy enough to create using pseudo-elements. It’s a nice and robust way to progressively enhance a design. This post is a summary of the technique and some of the possible appearances.

Demo: CSS drop-shadows without images

Known support: Firefox 3.5+, Chrome 5+, Safari 5+, Opera 10.6+, IE 9+

I’ll be looking mainly at a few details involved in making this effect more robust. Divya Manian covered the basic principle in her article Drop Shadows with CSS3 and Matt Hamm recently shared his Pure CSS3 box-shadow page curl effect.

After a bit of back-and-forth on Twitter with Simurai, and proposing a couple of additions to Divya’s and Matt’s demos using jsbin, I felt like documenting and explaining the parts that make up this technique.

The basic technique

There is no need for extra markup, the effect can be applied to a single element. A couple of pseudo-elements are generated from an element and then pushed behind it.

.drop-shadow {
  position: relative;
  width: 90%;
}

.drop-shadow:before,
.drop-shadow:after {
  content: "";
  position: absolute;
  z-index: -1;
}

The pseudo-elements need to be positioned and given explicit or implicit dimensions.

.drop-shadow:before,
.drop-shadow:after {
  content: "";
  position: absolute;
  z-index: -1;
  bottom: 15px;
  left: 10px;
  width: 50%;
  height: 20%;
}

The next step is to add a CSS3 box-shadow and apply CSS3 transforms. Different types of drop-shadow can be produced by varying these values and the types of transforms applied.

.drop-shadow:before,
.drop-shadow:after {
  content: "";
  position: absolute;
  z-index: -1;
  bottom: 15px;
  left: 10px;
  width: 50%;
  height: 20%;
  box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
  transform: rotate(-3deg);
}

One of the pseudo-elements then needs to be positioned on the other side of the element and rotated in the opposite direction. This is easily done by overriding only the properties that need to differ.

.drop-shadow:after{
  right: 10px;
  left: auto;
  transform: rotate(3deg);
 }

The final core code is as shown below. There is just one more addition – max-width – to prevent the drop-shadow from extending too far below very wide elements.

.drop-shadow {
  position: relative;
  width: 90%;
}

.drop-shadow:before,
.drop-shadow:after {
  content: "";
  position: absolute;
  z-index: -1;
  bottom: 15px;
  left: 10px;
  width: 50%;
  height: 20%;
  max-width: 300px;
  box-shadow :0 15px 10px rgba(0, 0, 0, 0.7);
  transform: rotate(-3deg);
}

.drop-shadow:after{
  right: 10px;
  left: auto;
  transform: rotate(3deg);
}

No Firefox 3.0 problems this time

Some pseudo-element hacks require a work-around to avoid looking broken in Firefox 3.0 because that browser does not support the positioning of pseudo-elements. This usually involves implicitly setting their dimensions using offsets.

However, as Divya Manian pointed out to me, in this case we’re only using box-shadow – which Firefox 3.0 doesn’t support – and Firefox 3.0 will ignore the position:absolute declaration for the pseudo-elements. This leaves them with the default display:inline style. As a result, there is no problem explicitly setting the pseudo-element width and height because it won’t be applied to the pseudo-elements in Firefox 3.0.

Further enhancements

From this base there are plenty of ways to tweak the effect by applying skew to the pseudo-elements and modifying the styles of the element itself. A great example of this was shared by Simurai. By adding a border-radius to the element you can give the appearance of page curl.

.drop-shadow {
  border-radius: 0 0 120px 120px / 0 0 6px 6px;
}

I’ve put together a little demo page with a few of drop-shadow effects, including those that build on the work of Divya Manian and Matt Hamm.

If you’ve got your own improvements, please send them to me on Twitter.

Emulating background image crop, background image opacity, background transforms, and improved background positioning. A few hacks relying on CSS pseudo-elements to emulate features unavailable or not yet widely supported by modern browsers.

Demos: Example CSS background image hacks

Pseudo-element hacks can fill some gaps in existing browser support for CSS features, without resorting to presentational HTML. In some cases, they even make it possible to emulate things that are not currently part of any W3C working draft, like background transforms and background image opacity.

Most of the hacks in this article tie in with the pseudo-element hack described in an earlier article – Multiple Backgrounds and Borders with CSS 2.1. That article already describes how to emulate multiple background support and its demo page shows several other uses of the basic principle. This article presents a few of those effects and applications in greater detail.

Emulating background-crop

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 8+

Demo: Pseudo background-crop

Background image cropping can be emulated in modern browsers using only CSS 2.1. The principle behind a pseudo background-crop is to apply a background-image to a pseudo-element rather than the element itself. One example would be to crop an image to display in the background. Another would be to crop an image sprite to display icons alongside text in links.

In several cases, using pseudo-elements may have advantages over existing, alternative techniques because it combines their strengths and avoids some of their weaknesses.

Google, Facebook, and Twitter all make use of empty DOM elements to crop dense sprites and display icons next to certain links in their interfaces. The alternative is not to use empty elements but be forced into using multiple images and/or to design sub-optimal image sprites that have their component images spaced out.

Pseudo-elements can be used in much the same way as empty DOM elements. This simultaneously eliminates the need for presentational HTML and doesn’t depend so heavily on an image sprite’s design. Using pseudo-elements for this purpose does have its own drawback – a lack of support in legacy browsers like IE6 and IE7. However, the technique will progressively enhance capable browsers while leaving a perfectly usable experience for incapable browsers.

Example code: cropping a sprite

This example shows how to crop icons that are part of a dense image sprite that uses a 16px × 16px grid. It uses a simple list and specifies a class for each type of action.

<ul class="actions">
  <li class="save"><a href="#">Save</a></li>
  <li class="delete"><a href="#">Delete</a></li>
  <li class="share"><a href="#">Share</a></li>
  <li class="comment"><a href="#">Comment</a></li>
</ul>

Styling can be applied to present this list in whatever way is needed. From that base, a pseudo-element can be created and then treated as you would an empty, inline DOM element (e.g. <span>).

In this case, the :before pseudo-element is used and sized to match the sprite’s grid unit. It could be sized to whatever dimensions are required to match a section of the sprite that needs to be cropped.

.actions a:before {
  content: "";
  float: left;
  width: 16px;
  height: 16px;
  margin: 0 5px 0 0;
  background: url(sprite.png);
}

.save a:before { background-position: 0 -16px; }
.delete a:before { background-position: 0 -32px; }
.share a:before { background-position: 0 -48px; }
.comment a:before { background-position: 0 -64px; }

Providing hover, focus, active, and “saved” states is just a case of declaring the correct background position in each case.

.save a:hover:before,
.save a:focus:before,
.save a:active:before {
  background-position: -16px -16px;
}

.saved a:before {
  background-position: -32px -16px;
}

Future alternatives

In the future, there will be other alternatives. Firefox 3.6 added -moz-image-rect to allow background images to be cropped. But this is not supported by other browsers and looks likely to be replaced by an alternative proposal (to use fragment identifiers) that is part of the CSS Image Values Module Level 3 specification. As far as I know, no stable release of any modern browser supports the use of fragment identifiers with bitmap images at the time of writing.

Emulating background-transform

Known support: Firefox 3.6+, Opera 10.5+, Safari 4+, Chrome 4+, IE 9+

Demo: Pseudo background-transform

Combining pseudo-elements and transforms makes it possible to emulate background transforms. A pseudo background-transform can be used to rotate, scale, and skew background images and sprites. There is no proposal for background-image transforms, so a pseudo-element hack is one way to emulate it.

Example: rotating a background image

The example of cropping sprites can be further developed by reducing the number of different images used in the sprite. Rather than applying transforms to images in a graphics package, they can be applied in the CSS.

The code to do this is relatively simple and might look something like:

.accordion a:before {
  content: "";
  float: left;
  width: 16px;
  height: 16px;
  margin: 0 5px 0 0;
  background: url(sprite.png) no-repeat 0 0;
}

.accordion.open a:before {
  transform: rotate(90deg);
}

To apply a transform to a more conventional background image (e.g., a large graphic sitting behind some content that doesn’t affect the positioning of other components) requires use of the positioning technique detailed in the article Multiple Backgrounds and Borders with CSS 2.1.

It involves setting the background image on a pseudo-element which is then positioned behind the content layer of an element using absolute positioning and z-index.

Example: mirroring a background image

There are instances when mirroring a background image might be desired. The approach is similar to that for rotating an image, but this time uses transform:scale().

Producing an exact mirror of an element or pseudo-element can be done using transform:scaleX(-1), transform:scaleY(-1), and transform:scale(-1,-1) to mirror along the x-axis, y-axis, and both axes, respectively.

The following code is an example of how a pseudo background-transform might be used for pagination links. A pseudo-element displays a single image (or region of a sprite) and is then mirrored. The image’s appearance is such that a rotation cannot produce the desired counterpart. Only a scale operation can do it.

.prev a:before,
.next a:before {
  content: "";
  float: left;
  width: 16px;
  height: 16px;
  margin: 0 5px 0 0;
  background: url(sprite.png) no-repeat 0 0;
}

.next a:before {
  float: right;
  margin: 0 0 0 5px;
  transform: scaleX(-1);
}

There is no support for this in IE 8. Even if you’re a fan of using IE filters to work around some missing CSS support, they won’t work on pseudo-elements.

Future alternatives

There don’t seem to be any future alternatives in any CSS working draft. For the moment, it looks like pseudo-element hacks will be needed to emulate effects like background transforms and background perspective without resorting to presentational HTML.

Emulating background-position

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 8+

Demo: Pseudo background-position

The CSS 2.1 specification limits the values of background-position to offsets from the left and top sides. It’s possible to emulate positioning a background image from the right and bottom sides by applying the background image to a pseudo-element and using it as an additional background layer.

This hack is easily combined with the other hacks in this article. More details on the pseudo background-position hack can be found in the article on Multiple Backgrounds and Borders with CSS 2.1.

Example code

In this example, a pseudo-element is created and placed behind the element’s content layer. The background image is 500px × 300px and declared for the pseudo-element, which is also given dimensions that match those of the image. Since the pseudo-element is absolutely positioned, it can be positioned from the bottom and right of the element using offsets.

#content {
  position: relative;
  z-index: 1;
}

#content:before {
  content: "";
  position: absolute;
  z-index: -1;
  bottom: 10px;
  right: 10px;
  width: 500px;
  height: 300px;
  background: url(image.jpg);
}

Future alternatives

There is a part of the CSS Backgrounds and Borders module working draft that describes an improvement to the background-position property to allow positions to be set from any side. At the moment, Opera 11 is the only stable release of a browser that has implemented it.

Emulating background-opacity

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 9+

Demo: Pseudo background-opacity

Changing the opacity of a pseudo-background is as simple as modifying the value of the opacity property. There is no IE 8 support for opacity and IE filters will not work on pseudo-elements.

Example code

This example code shows a pseudo-element being created and positioned behind the rest of the element’s content so as not to interfere with it. The pseudo-element is then sized to fit the element using offsets (but could be offset by other values or given an explicit size), given a background image, and has its opacity changed.

#content {
  position: relative;
  z-index: 1;
}

#content:before {
  content: "";
  position: absolute;
  z-index: -1;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: url(image.jpg);
  opacity: 0.7;
}

Notes

For now, and as far as I am aware, using CSS 2.1 pseudo-elements is the only widely supported (and backwards compatible) way to emulate background image crop, background transform, background opacity, and improved background positioning with semantic HTML.

Even when alternatives in CSS working drafts (e.g., the improved background-position and use of fragment identifiers) are widely implemented, pseudo-element background-image hacks will still have the advantage of letting you use other CSS properties like opacity, border-radius, border-image, box-shadow, transforms, etc., which may prove useful in certain situations. It can’t hurt to be aware of these options.

It’s worth mentioning that although you can only generate 2 pseudo-elements from a DOM element, in many cases you can easily use descendant elements to provide more pseudo-elements to play with. This idea was used to help create the rotated example on the CSS drop-shadows demo page and several of the CSS3 examples at the bottom of the pure CSS speech bubbles demo page.

Thanks to Mathias Bynens for reading and giving feedback on a draft of this article.

Future CSS implementations should allow for some form of responsive images via CSS alone. This is an early idea for how that might be done. However, a significant drawback is that it would not prevent both “mobile optimised” and larger size images from being requested at larger screen resolutions.

Note that the CSS presented here is not supported in any browsers at the time of writing.

This method relies on the use of @media queries, CSS3 generated content, and the CSS3 extension to the attr() function.

The principles are basically the same as those underpinning Filament Group’s work on Responsive Images. The source image is “mobile optimised” and the urls of larger size images are included using HTML data-* attributes.

<img src="image.jpg"
     data-src-600px="image-600px.jpg"
     data-src-800px="image-800px.jpg"
     alt="">

Using CSS @media queries you can target devices above certain widths. Within each media query block, images with larger alternatives can be targeted using an attribute selector.

CSS3 generated content allows you to replace the content of any element using the content property. At the moment, only Opera 10+ supports it. In CSS 2.1, the content property is limited to use with the :before and :after pseudo-elements.

By combining the content property with the CSS3 extension to attr(), you will be able to specify that an attribute’s value is interpreted as the URL part of a url() expression. In this case, it means you will be able to replace an image’s content with the image found at the destination URL stored in a custom HTML data-* attribute.

@media (min-device-width:600px) {
  img[data-src-600px] {
    content: attr(data-src-600px, url);
  }
}

@media (min-device-width:800px) {
  img[data-src-800px] {
    content: attr(data-src-800px, url);
  }
}

Fork the Gist

Issues

Unfortunately, there are a number of issues with this technique.

1. It doesn’t prevent multiple assets being downloaded at larger screen widths because network activity kicks in before CSS is applied. That means, for example, that desktop environments would make 2 HTTP requests for an image and have to load more assets than if they had been served only the larger image in the source.
2. It makes the assumption that wider screens are tied to better internet connections.
3. It forces authors to create and maintain multiple image sizes for each image.
4. At present, using the context menu (or drag and drop) to copy the image will result in the source file being copied and not the replacement image.
5. It doesn’t account for devices with different pixel densities.

A new image replacement technique was recently added to the HTML5 Boilerplate project. This post explains how it works and how it compares to alternative image replacement techniques.

[15 December 2012] This technique is no longer used in HTML5 Boilerplate. It’s been replaced by another, more reliable approach.

Here’s the CSS behind the recent update to the image replacement helper class in HTML5 Boilerplate. It has also made its way into the Compass framework.

.ir {
  font: 0/0 a;
  text-shadow: none;
  color: transparent;
}

What does each declaration do?

• font:0/0 a – a shorthand property that zeros out the font size and line-height. The a value acts as a very short font-family (an idea taken from the BEM implementation of this method). The CSS validator complains that using 0/0 in the shorthand font property is not valid, but every browser accepts it and this appears to be an error in the validator. Using font:0px/0 a passes validation but it displayed as font:0/0 a in the code that the validator flags as valid.
• text-shadow:none – makes sure that any inherited text shadow is removed for the text. This prevents the chance of any text shadow colors showing over the background.
• color:transparent – needed for browsers than don’t completely crush the text to the point of being invisible. Safari 4 (extremely rare) is an example of such a browser. There may also be mobile browsers than require this declaration. IE6/7/8 don’t recognise this value for color, but fortunately IE7/8 don’t show any trace of the text. IE6 shows a faint trace.

In the HTML5 Boilerplate image replacement helper, we’ve also removed any border and background-color that may be on the element. This makes it easier to use the helper class on elements like button or with links that may included background or border properties as part of a design decision.

Benefits over text-indent methods

The new technique avoids various problems with any text-indent method, including the one proposed by Scott Kellum to avoid iPad 1 performance problems related to large negative text indents.

• Works in IE6/7 on inline-block elements. Techniques based on text indentation are basically “broken”, as shown by this test case: http://jsfiddle.net/necolas/QZvYa/show/
• Doesn’t result in any offscreen box being created. The text-indent methods result in a box being drawn (sometimes offscreen) for any text that have been negatively or positively indented. It can sometimes cause performance problems but the font-based method sidesteps those concerns.
• No need to specify a text-alignment and hide the overflow since the text is crushed to take up no space.
• No need to hide br or make all fallback HTML display:inline to get around the constraints of using a text indentation. This method is not affected by those problems.
• Fewer styles are needed as a result of these improvements.

Drawbacks

No image replacement hack is perfect.

• Leaves a very small trace of the text in IE6.
• This approach means that you cannot use em units for margins on elements that make use of this image replacement code. This is because the font size is set to 0.
• Windows-Eyes has a bug that prevents the reading of text hidden using this method. There are no problems with all other screenreaders that have been tested. Thanks to @jkiss for providing these detailed results and to @wilto for confirming this technique works for JAWS 12 in IE 6/7/8 and Firefox 4/5/6.
• Like so many IR methods, it doesn’t work when CSS is loaded but images are not.
• Text may not be hidden if a visitor is using a user style sheet which has explicitly set important font-size declarations for the element type on which you have applied the IR class.

It’s worth noting that the NIR image replacement technique avoids these drawbacks, but lacks support in IE6/7.

Closing comments

I’ve been using this technique without significant problems for nearly a year, ever since Jonathan Neal and I used it in a clearfix experiment. The BEM framework also makes use of it for their icon components. The core idea was even proposed back in 2003 but the browser quirks of the day may have prevented wider use.

If you come across any problems with this technique, please report them at the HTML5 Boilerplate GitHub issue tracker and include a test case when appropriate.

Translations

• Nouvelle méthode de remplacement de texte par une image

I recently included the option to add a large cover image, like the one above, to my posts. The source image is cropped, and below specific maximum dimensions it’s displayed at a predetermined aspect ratio. This post describes the implementation.

• Demo: Flexible CSS cover images

Known support: Chrome, Firefox, Safari, Opera, IE 9+

Features

The way that the cover image scales, and changes aspect ratio, is illustrated in the following diagram.

The cover image component must:

• render at a fixed aspect ratio, unless specific maximum dimensions are exceeded;
• support different aspect ratios;
• support max-height and max-width;
• support different background images;
• display the image to either fill, or be contained within the component;
• center the image.

Aspect ratio

The aspect ratio of an empty, block-level element can be controlled by setting a percentage value for its padding-bottom or padding-top. Given a declaration of padding-bottom:50% (and no explicit height), the rendered height of the element will be 50% of its width.

.CoverImage {
  padding-bottom: 50%;
}

Changing that padding value will change the aspect ratio. For example, padding of 25% results in an aspect ratio of 4:1, padding of 33.333% results in an aspect ratio of 3:1, etc.

Maximum dimensions

The problem with using this aspect ratio hack is that if the element has a max-height declared, it will not be respected. To get around this, the hack can be applied to a pseudo-element instead.

.CoverImage:before {
  content: "";
  display: block;
  padding-bottom: 50%;
}

Now the main element can take a max-height. It should also clip the pseudo-element overflow.

.CoverImage {
  display: block;
  max-height: 300px;
  max-width: 1000px;
  overflow: hidden;
}

.CoverImage:before {
  content: "";
  display: block;
  padding-bottom: 50%;
}

This aspect ratio pattern is provided by the FlexEmbed component for SUITCSS. That component is primarily for responsive video embeds, but it’s flexible enough to be useful whenever you need an element rendered at a predetermined aspect ratio. It comes with modifiers for 2:1, 3:1, 16:9, and 4:3 aspect ratios. The cover image component can extend the FlexEmbed component.

<div class="CoverImage FlexEmbed FlexEmbed--2by1"></div>

Background image

The cover image is applied as a background image that is sized to cover the entire area of the element. This makes sure the image is clipped to fit the aspect ratio of the element.

.CoverImage {
  ...
  background-repeat: no-repeat;
  background-size: cover;
}

If you want different cover images for different instances of the component, they can be applied via the style attribute.

<div class="..." style="background-image: url(cover.jpg)"></div>

The image can be fully centered by using background positioning and block centering. This makes sure that the image is centered in the element, and that the element is centered within its parent (when it reaches the max-width value).

.CoverImage {
  ...
  background-position: 50%;
  background-repeat: no-repeat;
  background-size: cover;
  margin: 0 auto;
}

Final result

If you depend on the FlexEmbed module, the amount of additional code required is minimal. (See the demo for all the code, including the FlexEmbed dependency.)

/**
 * Requires: suitcss/flex-embed
 */

.CoverImage {
  background-position: 50%;
  background-repeat: no-repeat;
  background-size: cover;
  margin: 0 auto;
  max-height: 300px;
  max-width: 1000px;
}

<div class="CoverImage FlexEmbed FlexEmbed--3by1"
     style="background-image:url(cover.jpg)">
</div>

You can add further customizations, such as setting the accompanying background color, or providing a means to switch between the cover and contain keywords for background-size.

The responsive images spec is fantastic and covers a lot of use cases, but most of the time you’ll only need one: resolution switching using the `srcset` and `sizes` attributes.

Leung, Wing-Fai, 1970-

Hayden Library - QC173.6.C36 2019

London ; New York : Routledge, 2008

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