This morning hundreds of ladybirds were flying through the air and massing on the white walls of the house. I managed to get a few clear macro photographs.

The sun was shining and the ladybirds seemed to be attracted to anything white. I stuck a white T-shirt on and headed outside. Pretty soon I was covered in them and could pluck them from my shirt to get some close ups using my little Canon IXUS 60.

At some point a ladybird took off just before I tried to photograph it and I decided I’d try to capture that moment. A few minutes later I’d worked out that I could prompt one of the insects to walk up my finger like the stem of a flower, that they’d take off when they reached the tip, and that they took up a distinct posture just before their wing-case shot open.

The speed at which they prepare to take off, open their wings, and fly away is so quick that I just had to take the shot as soon as I saw a ladybird get into the “take-off position” and hope that I reacted fast enough to get a picture of the open wing-case.

This is a remix of another author’s idea of using CSS to make a classic model of our solar system. Here, I’ve relied on CSS pseudo-elements and generated content to render scale models of the solar system from simple markup of the raw information.

There are three demos for this experiment, which is based on Alex Giron’s original Our Solar System in CSS3.

• Basic demo: Reworking of original experiment
• Advanced demo: Scale model of the Solar System
• Advanced demo (keyboard support): Scale model of the Solar System

The basic demo uses only CSS and simple, semantic HTML to relatively faithfully reproduce Alex’s original result.

The advanced demo is a rough scale model of the Solar System. It uses the same HTML as the “basic demo” but makes extensive use of CSS pseudo-elements, generated content, and various bits of CSS3.

The advanced demo (keyboard support) is an attempt to provide keyboard support by introducing slight modifications to the HTML. I’ve commented out the animations in this version of the demo.

Why rework the original experiment?

I was curious to see if the same result could be achieved with simpler HTML, by relying on some newer CSS features.

I experimented a bit further with generated content, shadows, and the way the layout of the solar system is implemented. Doing this exposed me to some of the different ways modern browsers are implementing CSS3. I’ve described some of those differences and bugs below.

A scale model of the solar system

The main demo is a scale model of the solar system. It uses 3 different scales: one for the object diameters; one for the distance of the planets from the sun; and one for the orbital period of each planet.

Semantic HTML and Microdata

The HTML is a list where each list item contains a title and description. I’ve included some HTML Microdata to provide hooks for generated content.

<li id="earth" itemscope>
  <h2 itemprop="object">Earth
  <dl>
    <dt>Description</dt>
    <dd itemprop="description">Earth is an ocean planet. Our home world's abundance of water - and life - makes it unique in our solar system. Other planets, plus a few moons, have ice, atmospheres, seasons and even weather, but only on Earth does the whole complicated mix come together in a way that encourages life - and lots of it.</dd>
    <dt>Diameter</dt>
    <dd itemprop="diameter">12,755 <abbr title="kilometers">km</abbr></dd>
    <dt>Distance from sun</dt>
    <dd itemprop="distance">150×10<sup>6</sup> <abbr title="kilometers">km</abbr></dd>
    <dt>Orbital period</dt>
    <dd itemprop="orbit">365<abbr title="days">d</abbr></dd>
  </dl>
</li>

CSS pseudo-elements and generated content

Pseudo-elements are used to produce the planets, Saturn’s ring, the planet names, and to add the scale information.

Given that the scales only make sense when CSS is loaded it isn’t appropriate to have the scales described in the HTML. Both demos use the same HTML but only one of them is a rough scale model. Therefore, in the scale model demo I’ve used generated content to present the ratios and append extra information to the headings.

header h1:after {content:": A scale model";}
header h2:after {content:"Planet diameters 1px : 1,220 km / Distance from sun 1px : 7,125,000 km / Orbital period 1s : 4d";}

#earth dd[itemprop=diameter]:after {content:" (5px) / ";}
#earth dd[itemprop=distance]:after {content:" (22px) / ";}
#earth dd[itemprop=orbit]:after {content:" (91s)";}

Even more complex 3D presentations are likely to be possible using webkit-perspective and other 3D transforms.

Keyboard support

With a little modification it is possible to provide some form of keyboard support so that the additional information and highlighting can be viewed without using a mouse. Doing so requires adding block-level anchors (allowed in HTML5) and modifying some of the CSS selectors.

Modern browser CSS3 inconsistencies

This experiment only works adequately in modern browsers such as Safari 4+, Chrome 4+, Firefox 3.6+ and Opera 10.5+.

Even among the current crop of modern browsers, there are bugs and varying levels of support for different CSS properties and values. In particular, webkit’s box-shadow implementation has issues.

There are a few other unusual :hover bugs in Opera 10.5 (most obvious in the basic demo). It should also be noted that the :hover area remains square in all modern browsers even when you apply a border-radius to the element.

Border radius

There are also a few other peculiarities around percentage units for border radius. Of the modern browsers, a square object with a border-radius of 50% will only produce a circle in Safari 5, Chrome 5, and Firefox 3.6.

Safari 4 doesn’t appear to support percentage units for border radius at all (which is why the CSS in the demos explicitly sets a -webkit-border-radius value for each object). Safari 5 and Chrome 5 do support percentage units for this property. However, Chrome 5 has difficulty rendering a 1px wide border on a large circle. Most of the border simply isn’t rendered.

In Opera 10.5, if you set border-radius to 50% you don’t always get a circle, so I have had to redeclare the border-radius for each object in pixel units.

It appears that this is one aspect of Opera’s non-prefixed border-radius implementation that is incorrect and in need of fixing.

Box shadow

Safari 4’s inferior box-shadow implementation means that inset shadows are not rendered on the planet bodies. In addition, the second box-shadow applied to Saturn (used to separate the planet from its ring) is completely missing in Safari 4 as it does not support a spread radius value.

Safari 5 and Chrome 5 are better but still problematic. The second box-shadow is not perfectly round as the box-shadow seems to use the pseudo-element’s computed border-radius. Furthermore, Chrome 5 on Windows does not properly support inset box-shadow meaning that the shadow ignores the border-radius declaration and appears as a protruding square.

The use of box-shadow to separate Saturn from the ring isn’t strictly necessary. You can create the separated ring using a border but box-shadow cannot be applied in a way that casts it over a border. Another alternative would be to add a black border around the planet to give the illusion of space between itself and the ring, but all browsers display a few pixels of unwanted background colour all along the outer edge of the rounded border.

I wanted the ring to share the appearance of a shadow being cast on it. Opera 10.5 and Firefox 3.6 get it right. Both webkit browsers get it wrong.

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.

Applying conditional classnames to the html element is a popular way to help target specific versions of IE with CSS fixes. It was first described by Paul Irish and is a feature of the HTML5 Boilerplate. Despite all its benefits, there are still a couple of niggling issues. Here are some hacky variants that side-step those issues.

An article by Paul Irish, Conditional stylesheets vs CSS hacks? Answer: Neither!, first proposed that conditional comments be used on the opening html tag to help target legacy versions of IE with CSS fixes. Since its inclusion in the HTML5 Boilerplate project, contributors have further refined the technique.

However, there are still some niggling issues with the “classic” conditional comments approach, which Mathias Bynens summarized in a recent article on safe CSS hacks.

1. The Compatibility View icon is displayed in IE8 and IE9 if you are not setting the X-UA-Compatible header in a server config.
2. The character encoding declaration might not be fully contained within the first 1024 bytes of the HTML document if you need to include several attributes on each version of the opening html tag (e.g. Facebook xmlns junk).

You can read more about the related discussions in issue #286 and issue #378 at the HTML5 Boilerplate GitHub repository.

The “bubble up” conditional comments method

Although not necessarily recommended, it looks like both of these issues can be avoided with a bit of trickery. You can create an uncommented opening html tag upon which any shared attributes (so no class attribute) can be set. The conditional classes are then assigned in a second html tag that appears after the <meta http-equiv="X-UA-Compatible"> tag in the document. The classes will “bubble up” to the uncommented tag.

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta http-equiv="X-UA-Compatible"
          content="IE=edge,chrome=1">
    <meta charset="utf-8">
    <!--[if lt IE 7]><html class="no-js ie6"><![endif]-->
    <!--[if IE 7]><html class="no-js ie7"><![endif]-->
    <!--[if IE 8]><html class="no-js ie8"><![endif]-->
    <!--[if gt IE 8]><!--><html class="no-js"><!--<![endif]-->

    <title>Document</title>
  </head>
  <body>
  </body>
</html>

Fork the Gist

The result is that IE8 and IE9 won’t ignore the <meta http-equiv="X-UA-Compatible"> tag, the Compatibility View icon will not be displayed, and the amount of repeated code is reduced. Obviously, including a second html tag in the head isn’t pretty or valid HTML.

If you’re using a server-side config to set the X-UA-Compatible header (instead of the meta tag), then you can still benefit from the DRYer nature of using two opening html tags and it isn’t necessary to include the conditional comments in the head of the document. However, you might still want to do so if you risk not containing the character encoding declaration within the first 1024 bytes of the document.

<!DOCTYPE html>
<html lang="en">
<!--[if lt IE 7]><html class="no-js ie6"><![endif]-->
<!--[if IE 7]><html class="no-js ie7"><![endif]-->
<!--[if IE 8]><html class="no-js ie8"><![endif]-->
<!--[if gt IE 8]><!--><html class="no-js"><!--<![endif]-->
  <head>
    <meta charset="utf-8">
    <title>Document</title>
  </head>
  <body>
  </body>
</html>

Fork the Gist

The “preemptive” conditional comments method

Another method to prevent the Compatibility View icon from showing was found by Julien Wajsberg. It relies on including a conditional comment before the DOCTYPE. Doing this seems to help IE recognise the <meta http-equiv="X-UA-Compatible"> tag. This method isn’t as DRY and doesn’t have the character encoding declaration as high up in the document, but it also doesn’t use 2 opening html elements.

<!--[if IE]><![endif]-->
<!DOCTYPE html>
<!--[if lt IE 7]><html class="no-js ie6"><![endif]-->
<!--[if IE 7]><html class="no-js ie7"><![endif]-->
<!--[if IE 8]><html class="no-js ie8"><![endif]-->
<!--[if gt IE 8]><!--><html class="no-js"><!--<![endif]-->
  <head>
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <meta charset="utf-8">
    <title>Document</title>
  </head>
  <body>
  </body>
</html>

Fork the Gist

While it’s interesting to explore these possibilities, the “classic” method is still generally the most understandable. It doesn’t create invalid HTML, doesn’t risk throwing IE into quirks mode, and you won’t have a problem with the Compatibility View icon if you use a server-side config.

If you find any other approaches, or problems with those posted here, please leave a comment but also consider adding what you’ve found to the relevant issues in the HTML5 Boilerplate GitHub repository.

Thanks to Paul Irish for feedback and suggestions.

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

Two prominent medical experts — AIIMS director Dr Randeep Guleriya and Dr. Manish Suneja — flew into Ahmedabad on Friday following instructions from the Home Minister to guide local doctors

‘U.P. government crushing their rights’

8,023 doctors, 8,296 staff nurses, 4,105 paramedics and lab technicians have been identified as dedicated COVID-19 service providers