The device-pixel-ratio Use Case

Let’s start simply, with a fixed-width image that we want to adapt to varying device-pixel-ratios. To do this, we’ll use the first tool that the new specification gives us for grouping and describing image sources: the srcset attribute.

Say we have two versions of an image:

  • small.jpg (320 × 240 pixels)
  • large.jpg (640 × 480 pixels)

We want to send large.jpg only to users with high-resolution screens. Using srcset, we’d mark up our image like so:

<img srcset="small.jpg 1x, large.jpg 2x"
   src="small.jpg"
   alt="A rad wolf" />

The srcset attribute takes a comma-separated list of image URLs, each with an x descriptor stating the device-pixel-ratio that that file is intended for.

The src is there for browsers that don’t understand srcset. The alt, of course, is included for browsers that don’t render images at all. One element and three attributes gets us an image that looks crisp on high-resolution devices and efficiently degrades all the way down to text. Not too shabby!

The Fluid- And Variable-Sized-Image Use Cases

What that markup won’t do is efficiently squish and stretch our image in a fluid layout. Before addressing this fluid-image use case, we need a little background on how browsers work.

  • cropped-small.jpg (96 × 96 pixels)
  • cropped-large.jpg (192 × 192 pixels)
  • small.jpg (320 × 240 pixels)
  • medium.jpg (640 × 480 pixels)
  • large.jpg (1024 × 768 pixels)

How do we mark them up? Like so:

<picture>
   <source media="(min-width: 36em)"
      srcset="large.jpg  1024w,
         medium.jpg 640w,
         small.jpg  320w"
      sizes="33.3vw" />
   <source srcset="cropped-large.jpg 2x,
         cropped-small.jpg 1x" />
   <img src="small.jpg" alt="A rad wolf" />
</picture>

http://www.smashingmagazine.com/2014/05/14/responsive-images-done-right-guide-picture-srcset/#more-191697