Surfin' Safari

Improved support for high-resolution displays with the srcset image attribute

Posted by Dean Jackson on Monday, August 12th, 2013 at 11:33 am

WebKit now supports the srcset attribute on image (img) elements (official specification from the W3C). This allows you, the developer, to specify higher-quality images for your users who have high-resolution displays, without penalizing the users who don’t. Importantly, it also provides a graceful fallback for browsers that don’t yet support the feature.

See the new feature in action. Note that you’ll want a recent nightly build of WebKit. I’ve also included part of the demo inline below.

Example of the srcset attribute. The image contains a coloured striped pattern with some inline text that indicates which of the candidate images were selected.

As you may know, WebKit has supported the -webkit-image-set CSS function for more than a year now. It lets CSS properties that take images provide a list of candidate image urls, each with a modifier such as “2x”. This allows the browser to chose the best image for the user’s device. Before this feature, if you wanted to support high-resolution displays you had a few options, each with some downsides. You could duplicate your CSS. You could use JavaScript to query the device pixel ratio and update image resources after the page has loaded, possibly triggering multiple image loads. Or you could hard-code a higher-resolution image and thus penalize some users who were downloading more data than necessary. And if you were providing images of different resolution, there was the added pain of specifying related CSS properties such as the background-size or slices for border-image. It was annoying. Thankfully -webkit-image-set solved this by allowing you to write one simple rule and have the browser do the work of deciding which image to use and therefore which to download. (Hmmm… maybe -webkit-image-set deserves its own blog post! :) )

The srcset attribute on img is very similar to -webkit-image-set. In fact, you can think of it as the markup equivalent to the CSS feature. Like the list of candidate images in -webkit-image-set, you add a new attribute srcset to your image elements. It’s designed to be backwards compatible: browsers that don’t support the attribute ignore it, and continue to use the src value. Meanwhile, browser engines such as WebKit can look at the srcset and decide which image best suits your user’s device. In most cases you won’t need anything more than this:

<img src="normal-image.jpg" srcset="better-image.jpg 2x">

Notice the “2x” after “better-image.jpg”? That tells the browser that if you’re on a display with two or more device pixels per CSS pixel, it should use “better-image.jpg” instead of “normal-image.jpg”. And if you’re not on a high-resolution display, the browser will fall back to the value specified in src. You can also specify another candidate for 1x displays in srcset (as shown in the example).

You can read more about srcset in the official specification. Note that at the moment WebKit only supports the resolution modifiers (e.g. 1x, 2x, 3x). As with any new feature in WebKit there may be bugs, so please be on the lookout for anything that doesn’t behave as expected. In particular, you should verify that WebKit is downloading the minimal resources for a page, because that’s one of the goals of this feature.

Special thanks go to WebKit community members Romain Perier and Yoav Weiss who made important contributions (r153624, r153733) to this feature.

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