{"rowid": 269, "title": "Adaptive Images for Responsive Designs\u2026 Again", "contents": "When I was asked to write an article for 24 ways I jumped at the chance, as I\u2019d been wanting to write about some fun hacks for responsive images and related parsing behaviours. My heart sank a little when Matt Wilcox beat me to the subject, but it floated back up when I realized I disagreed with his method and still had something to write about.\n\nSo, Matt Wilcox, if that is your real name (and I\u2019m pretty sure it is), I disagree. I see your dirty server-based hack and raise you an even dirtier client-side hack. Evil laugh, etc., etc.\n\nYou guys can stomach yet another article about responsive design, right? Right?\n\nHalf the room gets up to leave\n\nWhoa, whoa\u2026 OK, I\u2019ll cut to the chase\u2026\n\nTL;DR\n\nIn a previous episode, we were introduced to Debbie and her responsive cat poetry page. Well, now she\u2019s added some reviews of cat videos and some images of cats. Check out her new page and have a play around with the browser window. At smaller widths, the images change and the design responds. The benefits of this method are:\n\n\n\tit\u2019s entirely client-side\n\timages are still shown to users without JavaScript\n\tyour media queries stay in your CSS file\n\tno repetition of image URLs\n\tno extra downloads per image\n\tit\u2019s fast enough to work on resize\n\tit\u2019s pure filth\n\n\nWhat\u2019s wrong with the server-side solution?\n\nResponsive design is a client-side issue; involving the server creates a boatload of problems.\n\n\n\tIt sets a cookie at the top of the page which is read in subsequent requests. However, the cookie is not guaranteed to be set in time for requests on the same page, so the server may see an old value or no value at all.\n\tServing images via server scripts is much slower than plain old static hosting.\n\tThe URL can only cache with vary: cookie, so the cache breaks when the cookie changes, even if the change is unrelated. Also, far-future caching is out for devices that can change width.\n\tIt depends on detecting screen width, which is rather messy on mobile devices.\n\tResponding to things other than screen width (such as DPI) means packing more information into the cookie, and a more complicated script at the top of each page.\n\n\nSo, why isn\u2019t this straightforward on the client?\n\nClient-side solutions to the problem involve JavaScript testing user agent properties (such as screen width), looping through some images and setting their URLs accordingly. However, by the time JavaScript has sprung into action, the original image source has already started downloading. If you change the source of an image via JavaScript, you\u2019re setting off yet another request.\n\nImages are downloaded as soon as their DOM node is created. They don\u2019t need to be visible, they don\u2019t need to be in the document.\n\nnew Image().src = url\n\nThe above will start an HTTP request for url. This is a handy trick for quick requests and preloading, but also shows the browser\u2019s eagerness to download images.\n\nHere\u2019s an example of that in action. Check out the network tab in Web Inspector (other non-WebKit development aids are available) to see the image requests.\n\nBecause of this, some client-side solutions look like this:\n\n\n\nwhere t.gif is a 1\u00d71px tiny transparent GIF.\n\nThis results in no images if JavaScript isn\u2019t available. Dealing with the absence of JavaScript is still important, even on mobile. I was recently asked to make a website work on an old Blackberry 9000. I was able to get most of the way there by preventing that OS parsing any JavaScript, and that was only possible because the site didn\u2019t depend on it.\n\nWe need to delay loading images for JavaScript users, but ensure they load for users without JavaScript. How can we conditionally parse markup depending on JavaScript support?\n\nOh yeah!