{"rowid": 219, "title": "Speed Up Your Site with Delayed Content", "contents": "Speed remains one of the most important factors influencing the success of any website, and the first rule of performance (according to Yahoo!) is reducing the number of HTTP requests. Over the last few years we\u2019ve seen techniques like sprites and combo CSS/JavaScript files used to reduce the number of HTTP requests. But there\u2019s one area where large numbers of HTTP requests are still a fact of life: the small avatars attached to the comments on articles like this one.\n\nAvatars\n\nMany sites like 24 ways use a fantastic service called Gravatar to provide user images. As a user, you can sign up to Gravatar, give them your e-mail address, and upload an image to represent you. Sites can then include your image by generating a one way hash of your e-mail address and using that to build an image URL. For example, the markup for the comments on this page looks something like this:\n\n<div>\n\t<h4><a href=\"http://allinthehead.com/\">\n\t\t<img src=\"http://www.gravatar.com/avatar.php?gravatar_id=13734b0cb20708f79e730809c29c3c48&size=100\" class=\"gravatar\" alt=\"\" height=\"100\" width=\"100\" />\n    Drew McLellan\n\t</a></h4>\n\t<p>This is a great article!</p>\n</div>\n\nThe Gravatar URL contains two parts. 100 is the size in pixels of the image we want.  13734b0cb20708f79e730809c29c3c48 is an MD5 digest of Drew\u2019s e-mail address. Using MD5 means we can request an image for a user without sharing their e-mail address with anyone who views the source of the page.\n\nSo what\u2019s wrong with avatars?\n\nThe problem is that a popular article can easily get hundreds of comments, and every one of them means another image has to be individually requested from Gravatar\u2019s servers. Each request is small and the Gravatar servers are fast but, when you add them up, it can easily add seconds to the rendering time of a page. Worse, they can delay the loading of more important assets like the CSS required to render the main content of the page.\n\nThese images aren\u2019t critical to the page, and don\u2019t need to be loaded up front. Let\u2019s see if we can delay loading them until everything else is done. That way we can give the impression that our site has loaded quickly even if some requests are still happening in the background.\n\nDelaying image loading\n\nThe first problem we find is that there\u2019s no way to prevent Internet Explorer, Chrome or Safari from loading an image without removing it from the HTML itself. Tricks like removing the images on the fly with JavaScript don\u2019t work, as the browser has usually started requesting the images before we get a chance to stop it.\n\nRemoving the images from the HTML means that people without JavaScript enabled in their browser won\u2019t see avatars. As Drew mentioned at the start of the month, this can affect a large number of people, and we can\u2019t completely ignore them. But most sites already have a textual name attached to each comment and the avatars are just a visual enhancement. In most cases it\u2019s OK if some of our users don\u2019t see them, especially if it speeds up the experience for the other 98%.\n\nRemoving the images from the source of our page also means we\u2019ll need to put them back at some point, so we need to keep a record of which images need to be requested. All Gravatar images have the same URL format; the only thing that changes is the e-mail hash. Storing this is a great use of HTML5 data attributes.\n\nHTML5 data what?\n\nData attributes are a new feature in HTML5. The latest version of the spec says:\n\n\n\tA custom data attribute is an attribute in no namespace whose name starts with the string \u201cdata-\u201d, has at least one character after the hyphen, is XML-compatible, and contains no characters in the range U+0041 to U+005A (LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).\n[\u2026]\nCustom data attributes are intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements. These attributes are not intended for use by software that is independent of the site that uses the attributes.\n\n\nIn other words, they\u2019re attributes of an HTML element that start with \u201cdata-\u201d which you can use to share data with scripts running on your site. They\u2019re great for adding small bits of metadata that don\u2019t fit into an existing markup pattern the way microformats do.\n\nLet\u2019s see this in action\n\nTake a look at the markup for comments again:\n\n<div>\n\t<h4><a href=\"http://allinthehead.com/\">\n\t\t<img src=\"http://www.gravatar.com/avatar.php?gravatar_id=13734b0cb20708f79e730809c29c3c48&size=100\" class=\"gravatar\" alt=\"\" height=\"100\" width=\"100\" />\n    Drew McLellan\n\t</a></h4>\n\t<p>This is a great article!</p>\n</div>\n\nLet\u2019s replace the <img> element with a data-gravatar-hash attribute on the <a> element:\n\n<div>\n\t<h4><a href=\"http://allinthehead.com/\" data-gravatar-hash=\"13734b0cb20708f79e730809c29c3c48\">\n    Drew McLellan\n\t</a></h4>\n\t<p>This is a great article!</p>\n</div>\n\nOnce we\u2019ve done this, we\u2019ll need a small bit of JavaScript to find all these attributes, and replace them with images after the page has loaded. Here\u2019s an example using jQuery:\n\n$(window).load(function() {\n\t$('a[data-gravatar-hash]').prepend(function(index){\n\t\tvar hash = $(this).attr('data-gravatar-hash')\n\t\treturn '<img width=\"100\" height=\"100\" alt=\"\" src=\"http://www.gravatar.com/avatar.php?size=100&gravatar_id=' + hash + '\">'\n\t})\n})\n\nThis code waits until everything on the page is loaded, then uses jQuery.prepend to insert an image into every link containing a data-gravatar-hash attribute. It\u2019s short and relatively simple, but in tests it reduced the rendering time of a sample page from over three seconds to well under one.\n\nFinishing touches\n\nWe still need to consider the appearance of the page before the avatars have loaded. When our script adds extra content to the page it will cause a browser reflow, which is visually annoying. We can avoid this by using CSS to reserve some space for each image before it\u2019s inserted into the HTML:\n\n#comments div {\n\tpadding-left: 110px;\n\tmin-height: 100px;\n\tposition: relative;\n}\n#comments div h4 img {\n\tdisplay: block;\n\tposition: absolute;\n\ttop: 0;\n\tleft: 0;\n}\n\nIn a real world example, we\u2019ll also find that the HTML for a comment is more varied as many users don\u2019t provide a web page link. We can make small changes to our JavaScript and CSS to handle this case.\n\nPut this all together and you get this example.\n\nTaking this idea further\n\nThere\u2019s no reason to limit this technique to sites using Gravatar; we can use similar code to delay loading any images that don\u2019t need to be present immediately. For example, this year\u2019s redesigned Flickr photo page uses a \u201cdata-defer-src\u201d attribute to describe any image that doesn\u2019t need to be loaded straight away, including avatars and map tiles.\n\nYou also don\u2019t have to limit yourself to loading the extra resources once the page loads. You can get further bandwidth savings by waiting until the user takes an action before downloading extra assets. Amazon has taken this tactic to the extreme on its product pages \u2013 extra content is loaded as you scroll down the page.\n\nSo next time you\u2019re building a page, take a few minutes to think about which elements are peripheral and could be delayed to allow more important content to appear as quickly as possible.", "year": "2010", "author": "Paul Hammond", "author_slug": "paulhammond", "published": "2010-12-18T00:00:00+00:00", "url": "https://24ways.org/2010/speed-up-your-site-with-delayed-content/", "topic": "ux"}