As originally proposed, the srcset attribute allowed developers to specify a list of sources for an image attribute, to be delivered based on the pixel density of the user’s display:
Not too scary, this markup. In plain English:
“Use low-res.jpg as the source for this img on low-resolution displays, and for any browser that doesn’t understand the srcset attribute. Use high-res.jpg as the source for this img on high-resolution displays in browsers that understand the srcset attribute.”
Things were starting to look scary, for a little while there. Due in part to high resolution devices, the average website is now nearly an entire megabyte of images. Now developers can target users on high-resolution displays with a high-resolution image source. Meanwhile, users on lower pixel density displays won’t be saddled with the bandwidth cost of downloading a massive high-resolution image, without seeing any benefit.
via Smashing Magazine.