Picture element and srcset attribute

Last update : July 5, 2014

Jason Grigsby outlined two years ago that there are two separate, but related requirements that need to be addressed regarding the use of the <img> element in responsive designs :

  1. enable authors to provide different resolutions of images based on different environmental conditions
  2. enable authors to display different images under different conditions based on art direction

Resolution Switching

When we handle an image for Retina displays, it makes sense to deliver a crispy, high resolution picture to the browser. When we send the same image to a mobile with a small screen or to a tablet connected with low speed, it’s efficient to save bandwidth and to reduce the loading and processing time by providing a small size picture.

In HTML, a browser’s environmental conditions are primarily expressed as CSS media features (orientation, max-width, pixel-density, …) and CSS media types (screen, print, …). Most media features are dynamic (a browser window is resized, a device is rotated, …). Thus a browser constantly responds to events that change the properties of the media features. Swapping images provides a mean to continue communication effectively as the media features change dynamically.

Art Direction

When we display an image about a subject (i.e. the brain structure) at a large size, it makes sense to show the context. When we display the same image on a small screen, it’s useful to crop it and to focus on a detail. This differentiation is ruled by art direction.


for small screens a detail looks better than the resized original (Wikipedia) picture


The @media query inside CSS or the media tag inside the link element are the key ingredients for responsive design. There are several tactics for deciding where to put breakpoints (tweak points, optimization points). As there are no common screen sizes, it doesn’t make sense to base the breakpoints on a particular screen size. A better idea is to look at the classic readability theory and to break the layout if the width of a column exceeds 75 characters or 10 words. These are the breakpoints. Vasilis van Gemert created a simple sliding tool to show the impact of language and font family on the text width.

Lucky responsive techniques

In the recent past, web developers relied on various techniques (CSS background images, Javascript libraries, semantically neutral elements, <base> tag switching, …) to use responsive images in their applications.  All of these techniques have significant limits and disadvantages (bypassing the browser’s preload scan, redundant HTTP requests, complexity, high processing time, …).  For all these reasons a standardized solution was wanted.

Possible responsive image solutions

The proposed solutions to deal with one or with both of the requirements for responsive images (“resolution switching” and “art-direction“) are the following :

  • <picture> element : addresses requirement #2 (the author selects the image series and specifies the display rules for the browser)
  • srcset and sizes attributes : addresses requirement #1 (the browser selects the image resolution based on informations provided by the author)
  • CSS4 image-set : addresses requirement #1 (the browser selects the images based on informations provided by the author)
  • HTTP2 client hints : addresses requirements #1 and #2 (the server select the images based on rules specified by the author)
  • new image format : addresses requirement #1 (there is only one image)

Responsive image standardization

On June 20, 2014, Anselm Hannemann, a freelance front-end developer from Germany, announced on his blog that the <picture> element and the attributes srcset and sizes are now web standards. The discussions and debates about the specification of a native responsive images solution in HTML lasted more than 3 years inside WHATWG, RICG and W3C.

The Responsive Images Community Group (RICG) is a group of developers working towards a client-side solution for delivering alternate image data based on device capabilities to prevent wasted bandwidth and optimize display for both screen and print. RICG is a community group of the World Wide Web Consortium (W3C). The group is chaired by Mathew Marquis of the Filament Group and has 362 participants, among them the Responsive Web Design pioneers Nicolas Gallagher, Bruce Lawson, Jason Grigsby, Scott Jehl, Matt Wilcox and Anselm Hannemann.

The RICG drafted a picture specification (editors draft July 1, 2014) with the new HTML5 <picture> element and the srcset and sizes attributes that extends the img and source elements to allow authors to declaratively control or give hints to the user agent about which image resource to use, based on the screen pixel density, viewport size, image format, and other factors.

Bruce Lawson was the first to propose the <picture> element and he has a degree of attachment to it. The srcset attribute was presented on the WHATWG mailing list by someone from Apple. At first, the majority of developers favored the <picture> element and the majority of implementors favored the srcset attribute. The W3C states how the priority should be given when determining standards:

In case of conflict, consider users over authors over implementors over specifiers over theoretical purity.

Both WHATWG and W3C included now the <picture> element and the srcset and sizes attributes to the HTML5 specification. The links are given below :

The <picture> element

The use of the <picture> element is shown in the following code examples :

 <source srcset="brain-mobile.jpg, brain-mobile-x.jpg 2x">
 <source media="(min-width: 480px)" srcset="brain-tablet.jpg, 
    brain-tablet-hd.jpg 2x">
 <source media="(min-width: 1024px)" srcset="brain-desktop.jpg, 
    brain-desktop-hd.jpg 2x">
 <img src="brain.jpg" alt="Brain Structure">

With a mobile-first approach, the image “brain-mobile.jpg” is rendered by default, the image “brain-tablet.jpg” is rendered if the user screen is at least 480px wide, and “brain-desktop.jpg” is rendered if the user screen is at least 1024px wide. The image “brain.jpg” is destinated to those browsers who don’t understand the <picture> element. The second URL in the srcset attribute is paired with the string 2x, separated by a space, that targets users with a high-resolution display (like the Retina with a pixel density 2x).

<source sizes="100%" srcset="brain-mobile.jpg 480w, 
brain-tablet.jpg 768w, brain-desktop.png 1024w">
<img src="brain.jpg" alt="Brain Structure">

In the second example the sizes attribute is used to let the image cover all the width of the device (100%), regardless of its actual size and pixel density. The browser will automatically calculate the effective pixel density of the image and choose which one to download accordingly.

The four images brain-mobile.jpg, brain.jpg, brain-tablet.jpg and brain-desktop.jpg not only have different dimensions, but may  also have different content. This way authors are enabled to display different images under different conditions, based on art direction.

The <picture> element should not be confused with the HTML5 <figure> element which represents some flow content. The <figure> element is able to have a caption, typically the <figcaption> element.

   <figcaption>Brain Structure</figcaption> 
   <img src="brain.jpg" alt="Brain Structure" width="320"/>

The sizes syntax is used to define the size of the image across a number of breakpoints. srcset then defines an array of images and their inherent sizes.

The srcset attribute

The srcset is a new attribute for use in the <img> elements. Its value is a comma separated list of images for the browser to choose from. An simple example is shown below :

<img srcset="brain-low-res.jpg 1x, brain-hi-res 2x, width="320"
alt="Brain Structure">

We tell the browser that there is an image to be rendered at 320 CSS pixels wide. If the device has a normal 1x screen, a low resolution image 320 x 240 pixels is loaded. I the device has a pixel ratio of 2 or more, a higher resolution image 640 x 480 pixels is requested from the server by the browser.

Here comes a second example :

<img src="brain.jpg" sizes="75vw" 
srcset="brain-small.jpg 320w, brain-medium.jpg 640w, 
brain-large.jpg 1024w,brain-xlarge.jpg 2000w" 
alt="Brain Structure">

The srcset attribute tells the browser which images are available with their respective pixel widths. It’s up to the browser to figure out which image to load, depending on the viewport width, the pixel ratio, the network speed or anything else the browser feels is relevant.

The sizes attribute tells the browser that the image should be displayed at 75% of the viewport width. The sizes attribute is however more powerful than indicating default length values. The format is :

sizes="[media query] [length], [media query] [length] ... etc"

Media queries are paired with lengths. Lengths can be absolute (pixel, em) or relative (vw). The next exampe shows a use-case:

<img src="brain.jpg" 
sizes="(min-width:20em) 240px,(min-width:48em) 80vw, 65vw"
srcset="brain-small.jpg 320w, brain-medium.jpg 640w, 
brain-large.jpg 1024w,brain-xlarge.jpg 2000w" 
alt="Brain Structure">

We tell the browser that in viewports between 0 and 20 em wide the image should be displayed  240 pixels wide, in viewports between 20 em and 48 em wide the image should take up 80% of the viewport and in larger viewports the image should be 65% wide.

Can I use responsive images ?

The support of the <picture> element and the srcset and sizes attributes in the various browsers can be checked at the “Can I Use” website. This site was built and is managed by Alexis Deveria, it provides up-to-date support tables of front-end web technologies on desktop and mobile browsers.

Support of the picture element in browsers

Support of the picture element in browsers (Can I Use Website)

Support of the srcset attribute in browsers

Support of the srcset attribute in browsers (Can I User Website)

Actually the <picture> element is only supported by Firefox version 33. The srcset attribute is only supported by Firefox versions >32, Chrome versions > 34, Safari version 8 and Opera versions > 22.


The poor support of the <picture> element and the srcset attribute in actual browsers does not mean that you have to wait before implementing responsive images in your website. Scott Jehl from Filament Group developped a great polyfill called PictureFill supporting the <picture> element and the srcset and sizes attributes.

Initialization code :

// Picture element HTML5 shiv
document.createElement( "picture" );
<script src="picturefill.js" async></script>

<picture> code :

<!--[if IE 9]><video style="display: none;"><![endif]-->
<source srcset="brain-xx.jpg" media="(min-width: 1000px)">
<source srcset="brain-x.jpg" media="(min-width: 800px)">
<source srcset="brain.jpg">
<!--[if IE 9]></video><![endif]-->
<img srcset="brain.jpg" alt="Brain Structure">

If JavaScript is disabled, PictureFill only offers alt text as a fallback. PictureFill supports SVG and WebP types on any source element, and will disregard a source if its type is not supported. To support IE9, a video element is wrapped around the source elements using conditional comments.scrset code :

<img sizes="(min-width: 40em) 80vw, 100vw"
srcset="brain-s.jpg 375w,brain.jpg 480w,brain-x.jpg 768w" 
alt="Brain Structure">

The PictureFill syntax is not quite the same as the specification. The fallback src attribute was intentionally removed to prevent images from being downloaded twice.

CSS4 image-set

By using the CSS4 image-set function, we can insert multiple images which will be set for normal and high-resolution displays. The image-set function is declared within the background-image property, while the background URL is added within the function followed by the resolution parameter (1x for normal display and 2x is for high-res display), like so :

.selector { 
 background-image: image-set(url('image-1x.jpg') 1x, 
 url('image-2x.jpg') 2x); 

The CSS4 image-set function is also trying to deliver the most appropriate image resolution based on the connection speed. So, regardless of the screen resolution, if the user accesses the image through a slow Internet connection, the smaller-sized image will be delivered.

CSS4 image-set is still experimental. It is only supported in Safari 6 and Google Chrome 21 where it is prefixed with -webkit.

HTTP2 client hints

The responsive image standards leave the burden to create images at appropriate sizes, resolutions and formats to the web developer. Client hints are a way to offload this work to the server. Client hints are HTTP headers that give the server some information about the device and the requested resource. Ilya Grigorik, web performance engineer and developer advocate at Google, submitted in December 2013 an Internet Draft “HTTP client hints” to the Internet Network Working Group of the Internet Engineering Task Force (IETF). The draft specifies two new headers for the HTTP 2.0 version : CH-DPR for device pixel ratio and CH-RW for resource width. A server-side script will generate the best image for the requesting device and deliver it.

New image formats

There are some new image formats like JPEG2000, JPEG XR and WebP that generate higher quality images with smaller file sizes, but they aren’t widely supported. JPEG 2000 is scalable in nature, meaning that it can be decoded in a number of ways. By truncating the codestream at any point, one may obtain a representation of the image at a lower resolution. But the web already has this type of responsive image format, which is progressive JPEG, if we get the browsers to download only the neccesary bytes of the picture (i.e. with the byte range HTTP header). The main problem is that the new image formats will take long to implement and deploy, and will have no fallback for older browsers.


The following list provides links to websites with additional informations about <picture>, srcset, PictureFill and related topics :

Responsive iFrames and Image Maps

Last update : June 27, 2014

Some HTML elements don’t work with responsive layouts. Among these are iFrames, which you may need to use when embedding content from external sources. Other elements are Image Maps which are lists of coordinates relating to a specific image, created in order to hyperlink areas of the image to different destinations.

Responsive iFrames

When you embed content from an external source with an iFrame, you must include width and height attributes. Wihtout these parameters, the iframe will disappear because it would have no dimensions. Unfortunaltely you can’t fix this in your css style sheet.

To make embedded content responsive, you need to add a containing wrapper around the iframe :
<div class="iframe_container">
<iframe src="http://www.yoursite.com/yourpage.html" width="640" height="480">

The containing wrapper is styled with the .iframe_container class in the style sheet :
.iframe_container {
position: relative;
padding-bottom: 75%;
height: 0;
overflow: hidden;

Setting the position to relative lets us use absolute positioning for the iframe itself. The padding-bottom value is calculated out of the aspect ratio of the iFrame, which in this case is 480 / 640 = 75%. The height is set to 0 because padding-bottom gives the element the height it needs. The width will automatically resize with the responsive element included in the wrapping div. Setting overflow to hidden ensures that any content flowing outside of this element will be hidden from view.

The iFrame itself is styled with the following CSS code :
.iframe_container iframe {
position: absolute;
left: 0;
width: 100%;
height: 100%;

Absolute positioning must be used because the containing element has a height of 0. The top and left properties position the iFrame correctly in the containing element. The width and height properties ensure that the iFrame takes up 100% of the space used by the containing element set with padding.

Responsive Image Maps

Image maps are a co-ordinate representations of images in sections mostly in rectangular, poly and circle format. According to the specs percent values can be used for coordinates, but no major browsers understand them correctly and all interpret coordinates as pixel coordinates. The result is that image maps applied to responsive images don’t work as expected when images are resized. It’s necessary to recalculate the area coordinates to match the actual image size.

There are different solutions available to make Image Maps responsive :

The following demo shows a responsive Image Map embedded in a responsive iFrame :


The list below shows links to websites sharing additional informations about responsive iFrames and Image Maps :


Responsive video

Last update : June 27, 2014

Responsive webdesign needs not only adaptive images, but also fluid videos. The pioneers in emdedding responsive videos in webpages are Chris Coyier and Dave Rupert. They created in 2011 FitVids, a lightweight, easy-to-use jQuery plugin for reactive width video embeds. FitVids automates the Intrinsic Ratio Method by Thierry Koblentz to achieve fluid width videos in your responsive web design. The source code is available at Github.

Here is a list of more useful links to contributions about responsive video :

Modal dialogs

With modal dialogs (overlays), users don’t have to deal with multiple windows. When a modal window opens, it opens inside the current page and users don’t have to deal with extra windows popping up.

When a modal dialog is shown, the content beneath the overlay cannot be acted upon until the overlay is dismissed. Modal overlays don’t allow you to refer back and forth between two sources of information, or move fluidly between two actions.

Today it is good practice to darken the parent window to provide a visual indicator to the user that the main window is inactive. This technique is usually called Lightbox.

The following list provides some links to blogs about the con’s and pro’s of modal overlays:

Responsive javascript slideshows

Last update : July 5, 2014

Early 2006, Lokesh Dhakar introduced Lightbox, a unobtrusive script used to display large images using modal dialogs over a black faded background. Six years later, there were hundred of scripts available to create slideshows, carousels, sliders and image galleries on the web. I evaluated the following responsive javascript slideshows for different projects:

Today, some of these scripts are outdated and I included them only for historical reasons. Some scripts are standalone, but most are based on jQuery, a fast, small and feature-rich JavaScript library. Most of them are also available as WordPress plugin.

My favorite script is the …

I use the following code :


See the demo

The following list provides links to websites with additional informations about slideshows and related topics :

RESS: Responsive Design + Server Side Components

Last update : June 26, 2014

In Responsive Web Design implementations, Web URLs are consistent across devices and adapt their content based on the capabilities of the browser in which they are displayed.

Server side solutions, on the other hand, only send what a client needs. But server-side solutions generally rely on user agent redirects to device-specific code templates. Each device class that warrants adaptation needs its own set of templates and these templates may ultimately contain duplicative code that actually applies to every class of device.

WordPress Post Formats

Post formats have been around since WordPress 3.1. They specify the format applied to a content. Post formats are different from Post Types which specify the type of content : post, page or maybe a custom type like books or products.

The following Post Formats are supported :

  • standard : post with title, date and author
  • aside : post without title and data, with the hint “aside”
  • link : post without title and data, with the hint “link”
  • gallery : first picture of a native gallery and number of pictures in the gallery
  • status : format like Twitter, with a gravatar
  • quote : post without title and data, with the hint “quote”
  • image : special format for photoblogging
  • video : special format for videos
  • audio : special format for audios
  • chat : special format for chat logs

Post Formats

The WordPress theme Twenty Ten supports the standard, aside and gallery format. The new theme Twenty Eleven supports all formats, but video, audio and chat need to be activated by adding them to the post formats array in the theme file functions php :

// Add support for a variety of post formats
add_theme_support( 'post-formats', array( 'aside', 'link', 'gallery', 'status', 'quote', 'image', 'chat', 'video', 'audio' ) );

They need to be styled further in the corresponding css-file.

To add the Post Formats in a child theme, you must ensure that the childtheme setup occurs after the parenttheme setup. This is the code :

function childtheme_setup(){ add_theme_support( 'post-formats', array( 'aside', 'link', 'gallery', 'status', 'quote', 'image', 'chat', 'video', 'audio' ) ); add_action( 'after_setup_theme', 'childtheme_setup' );

The following list shows links to useful tutorials and documentation about WordPress Post Formats :

Hereafter are some older posts about WordPress themes and templates :

Adaptive and context-aware images

Last update : July 1, 2014

Each image has an innate, original height and width that can be derived from the image data. This derived height and width information is content, not layout, and should therefore be rendered as HTML <img> element attributes. To override height and width for adaptive images, CSS is the best approach.

Context-aware images in responsive web design are created by declaring a 100% width in CSS :

img {width : 100%}

Modern browsers keep the image’s proportions intact. To render a context-aware image at its native dimensions and to resize it only if it exceeds the width of its container, the classic solution is the CSS code :

img {max-width :100%}

But scaling down images is not sufficient. Sending huge image files to low-performance devices with narrow screens is not very clever. The early HTML image tags had a lowscr attribute which is no longer supported by modern browsers.

Sending the right-sized image to devices without wasting bandwith is one of the challenges in responsive web design. The main problem is that the HTML img tag has only one source attribute today.

The different techniques proposed in the recent past by the pioneers of context-aware images, listed at the end of this post, can be grouped as follows :

  1. client-side current technology solutions
  2. server-side current technology solutions
  3. standardized future-technology solutions

Client-side current technology solutions :

  • CSS with background-images controlled by media-queries : Harry Roberts
  • dirty script to load temporary images and altering the image source path before handing it to the DOM parsing (1×1 pixel transparent GIF and <noscript> tag) : Jake Archibald, (Mairead Buchan, Antti Peisa, Vasilis van Gemert)
  • dynamic modification of the base tag : Scott Jehl
  • CCS3 content & attr() : Nicolas Gallagher

Server-side current technology solutions :

  • php-script dealing with AJAX requests : James Fairhurst
  • php-script dealing with image arrays : Craig Russel
  • php-script dealing with cookies : Matt Wilcox, Keith Clark, Scott Jehl, (Mark Perkins, Andy Hume)
  • service to resize images – tinySCR, now Sencha.io.SCR : James Pearce, (Graham Bird, Andrea Trasatti)
  • service to adapt mobile content to cell-phones with Opera Mini pre-installed : Opera Software ASA

Standardized future-technology solutions :

  • new html attribute pointing to a list of sources : Dominique Hazael-Massieux, Anselm Hannemann
  • new picture tag : Bruce Lawson, Jake Archibald, Nicolas Gallagher
  • progressive resolution-enhanced streaming images
  • new HTTP headers for content negotiation

Problems :

“Starting with small images for mobiles and upgrade to large images for desktops, without loading both and working with all browsers” is the goal of responsive design with context-aware images. Until now there is no solution that meets this objectif.

The common problems are listed below :

  • dynamic image tags : double downloads with external scripts ; race problems with internal scripts in some browsers ; inconsistencies when diffrent scripts change the base tag
  • temporary image : without javascript the image never loads
  • noscript tag : some browsers cannot manipulate the noscript tag
  • CSS generated content : only supported in Opera10+
  • cookies : no effect when cookies are disabled
  • same URL :  caching fails
  • php scripts : CDN routing not possible
  • device detection : not reliable for all browsers

Some pioneers in the field of “adaptive images” are presented hereafter :

Scripts, programs and tools :

Contributions to W3C and drafts by W3C :

Blogs :

more contributors :

Google Chrome Frame

Google Chrome Frame is an open source plug-in that seamlessly brings Google Chrome’s open web technologies (for instance the canvas tag) and speedy JavaScript engine to Internet Explorer (IE 6, 7, 8, or 9).

Enabling Google Chrome Frame is simple. For most web pages, all you have to do is add a single tag to your pages and detect whether your users have installed Google Chrome Frame.

  • If Google Chrome Frame is not installed, you can direct your users to an installation page.
  • If Google Chrome Frame is installed, it detects the tag you added and works automatically.

Semantic Web

Last Update : October 7, 2012

The Semantic Web is a collaborative movement led by the international standards body W3C. The Semantic Web is a Web of Data, as opposed to the existing Web of Documents. The goal of the Web of Data is to enable computers to do more useful work and to develop systems that can support trusted interactions over the network.

The Web of Data is empowered by new technologies such as RDFa (Resource Description Framework-in attributes), SPARQL, OWL (Web Ontology Language), SKOS (Simple Knowledge Organization System), Microdata and Open Graph.

HTML (HyperText Markup Language) remains still the main markup language for displaying web pages and other information that can be displayed in a web browser.

Semantic HTML refers to the semantic elements and attributes of HTML (h1, h2, …, p, …), as opposed to the presentational HTML elements and attributes (center, font, b, …). The acronym POSH was coined in 2007 for semantic HTML, as a shorthand abbreviation for “plain old semantic HTML”.

HTML5 introduced a few new structural elements :

  • <header> : this tag replaces the <div class=”header”>, commonly used in the past by most designers. The header element contains introductory information to a section or page.
  • <footer> : same as above, it’s the well known <div class=”footer”>. The footer element is for marking up the baseline of the current page and of each section contained in the page.
  • <nav> : replacement for <div class=”navigation”>. The nav element is reserved for the primary navigation. Not all link groups in a page or section need to be contained within the <nav> element.
  • <section> : this is the replacement for the generic flow container <div> when it contains related content. <div> is a block-level element with no additional semantic meaning, whereas <section> is a sectioning element which has normally a header and a footer and represents a generic document or application section.
  • <article> : the <article> element represents a portion of a page or section which can stand alone and makes sense even outside the context of the page. Like <section>, an <article> generally has a header and a footer. You should avoid nesting an <article> inside another <article>.

HTML5 tag <aside>

  • <aside> : this tag is used to represent content that is related to the surrounding content within an section, article or web page, but could still stand alone in its own right. (see figure at right). This type of content is often represented in sidebars.
  • <hgroup> : A special header element that must contain at least two <h1>-<h6> tags and nothing else. It’s a group of titles with subtitles. Make sure to maintain the <h1> – <h6> hierarchy.

RDFa is a W3C Recommendation that adds a set of attribute-level extensions (rich metadata) to web documents. RDFa 1.1 was approved in June 2012. It differs from RDFa 1.0 in that it no longer relies on the XML-specific namespace mechanism, but ca be used with non-XML document types such as HTML 4 or HTML 5. eRDF is an alternative to RDFa. SPARQL is an RDF query language. On 15 January 2008, SPARQL 1.0 became an official W3C Recommendation. OWL is a family of knowledge representation languages for authoring ontologies. An ontology formally represents knowledge as a set of concepts within a domain in computer science and information science, and the relationships among those concepts. Ontologies are the structural frameworks for organizing information and are used, among others, in artificial intelligence. SKOS is a family of formal languages designed for representation of of structured controlled vocabulary (thesauri, classification schemes, taxonomies, …). Microdata is a WHATWG specification used to nest semantics within existing content on web pages. The Open Graph protocol, originally created by Facebook, enables any web page to become a rich object in a social graph.

All these technologies help computers such as search engines and web crawlers better understand what information is contained in a web page, providing better search results for users.

Another set of simple, structured open data formats, built upon existing standards, is Microformats. One difference with the other semantic technologies is that Microformats is designed for humans first and machines second.

The following list provides links to some useful blogs and tutorials about the semantic web: