The road to responsive images


Last week’s W3C editor’s draft brought us one step closer to a workable specification for responsive images. Anselm Hannemann explores how the draft spec came about, the problems that remain to be solved – and why we need a specification in the first place

A bit of history

In case you’ve missed the debate about responsive images, here’s a recap. Many people (including me) have written articles on the topic, so this is just a brief overview:

  1. I requested a native HTML solution for responsive images from WHATWG in July 2011.
  2. The topic disappeared until November 2011.
  3. Other people started investigating and the topic went public, without response from W3C/WHATWG.
  4. In February 2012, W3C formed a community group to discuss the topic further.
  5. In February 2012, WHATWG discussed another approach using the srcset attribute, confusing all of us because it diverged from our proposed solution. This proposal went public – prompting further debate – in May 2012
  6. After further discussions among both W3C and WHATWG, it finally became clear in August 2012 that the W3C wants a picture element including srcset.
Why we need a specification for responsive images: just a few of the devices available for viewing images online

The W3C first draft

The first draft of the specification has now been published. The good news is that W3C, WHATWG and most of the list subscribers agree with this draft in outline, so discussion has moved from the underlying approach to the details and remaining gaps in the spec.

Top: the screen of an iPad 2. At 132 ppi resolution, you can clearly see the pixels. Bottom: the high-resolution screen of the iPad 3 looks much smoother to the eye

The goal of responsive images

If you’re still unclear about why we need a specification for responsive images, here’s what it allows you to do (also check out the official goals):

  • Respond to different screen pixel width/height in HTML.
  • Respond to different screen pixel densities (low-res/high-res like Apple’s Retina displays).
  • Respond to users zooming in on images (for example, in a lightbox).
  • Provide user agents with the information they need to select the most appropriate image source in low-bandwidth situations, via media queries (not yet available).
  • Preserve separation of content markup and styling.
  • Provide a purely client-side solution that can include JavaScript, but doesn’t require it.
  • Provide different crops of an image according to the viewport or device through which it is viewed (for example, a black-and-white picture for E Ink readers).
The goal of responsive images
How high-resolution screens work. On the left, four device pixels on a standard-resolution screen; on the right, the 16 device pixels that result in an image of the same size when the result is displayed on a high-resolution screen

How you will use responsive images

The current specification enables you to do one of two things:

  1. Specify different resolutions of the same image using the srcset attribute. You don’t have to care about media queries and only provide images for 1x (normal resolution), 1.5x (higher resolution), and 2x (Retina resolution). The user agent does the rest.
  2. Use different images for different resolutions. You specify the media queries, targeting different resolutions or different device types and serve specific images to each user. More granular, but more effort.

The picture element will have this syntax:

  1. <picture>
  2. <source media = “(min-width: 45em)” srcset = “large-1.jpg 1x, large-2.jpg 2x”>
  3. <source media = “(min-width: 18em)” srcset = “med-1.jpg 1x, med-2.jpg 2x”>
  4. <source srcset = “small-1.jpg 1x, small-2.jpg 2x”>
  5. <img src = “small-1.jpg” alt = “my alt-text” width = “200” height = “100”>
  6. </ Hình ảnh>

This is a basic example of how to write responsive images. As you can see, the new element is backwards-compatible, due to the fact you can integrate the common img element in the picture element. That means you always have a fallback for older browsers not supporting the picture element – and therefore, that can make use of the picture element right now. However, I would not recommend using sytax specified in a first draft for production use.

You can combine srcset with media queries to target specific screen sizes and let the user agent decide if the user gets a low-res or a high-res image. This makes the syntax ultra-flexible in targeting nearly any edge case for responsive images you might ever encounter.

How you will use responsive images
The file sizes you will need when creating an image for screens with different pixel densities. The image should display at the same size, no matter on which device it is viewed: the increase in density just results in a sharper image

Browser requirements

A few things are necessary to make the picture element work. These are browser requirements only, so I’ll only summarise the key points here:

  • The user agent should fetch the appropriate asset in one single request.
  • The user agent should be able to override developer values with user settings (for example, when a developer says to download the high-res image but the user has set their user agent to ‘low bandwidth’, indicating that they want – and will get – a low-res variant of the image).

Currently unsolved problems

Since the first draft came out a few days ago, there have been ongoing discussions on the WHATWG and W3C mailing lists. Luckily, most of the major issues seem to have been addressed. Most of the remaining unsolved problems are relatively trivial, so I won’t go into detail here. However, there is one important exception:

It is still unclear how to define alternative content for a picture element. On the mailing lists, one suggestion is for a full rich-text fallback inside the element (as child nodes). Another is to use a simple alt-attribute on either the picture element or the source element. As this poses important accessibility issues, it may take time to get this sorted.

Feedback needed!

Browser vendors have yet to provide feedback on this proposal. It is unclear if they are interested in implementing the specifications, or see implementation problems overlooked by the W3C. Bruce Lawson of Opera has responded by requesting more feedback.

So, dear readers, we really need your feedback on this topic: please provide it – or at least help to spread the word!

Thank you!

Finally, I want to say thank you to everyone who has helped advance the debate surrounding responsive images. I’d like to mention some specific people again: Bruce Lawson, who originally proposed the picture element; Mathew Marquis, who heads up the W3C effort, and who co-wrote the specification; and Matthew Wilcox who invested much time in evangelising the topic, providing feedback and searching for better solutions.

Thanks also to Rodney Rehm and Marc Hinse for proof reading this article!