No, CSS isn't always faster

Recently, a post appeared on Hacker News that was yet another exploration of crazy things that can be achieved with CSS.  Like many others, this example was logos rendered using HTML & CSS.  I quickly commented that I didn't feel we needed more of these examples which, while cool, do almost nothing to advance web development in general. (Note: as Nicolas Gallagher, mentioned in the comments, his article clearly states that these his icons are an experimentation and shouldn't be deployed in a real-world environment; apologies if the initial text implied that these demonstrations are endorsements of the "use this in the wild" approach).  I suggested that the people involved in such endeavors would be better suited devoting their efforts to demonstrating tips and techniques applicable to the real world.

While I'm sure I could devote an entire post to why CSS is the wrong tool for this job (see the "related reading" section below for a great argument there), one response to my comment in particular caught my attention:

A CSS logo would be useful for performance issues and overall page rendering speed. A CSS animated logo, such as the Atari example, even moreso than a Flash / JS counterpart. CSS renders more quickly than images, and definitely more quickly than embedded Flash.

It's time for this misinformed, dogmatic approach to web performance to end.  

When Steve Souders came out with his book, High Performance Websites, and the accompanying YSlow tool, the race to reduce HTTP requests was on! Several years later, it's somewhat unfortunate that in all of the great advice that Steve doled out, many people never make it past this first bullet point. It's also unfortunate that this rule set has defined website performance for the masses primarily in terms of network-related bottlenecks.  It's some to start thinking about performance from a holistic point of view; and not isolating things like network performance and elevating them to higher importance than other concerns.

Note: I mention Steve Souders' work here here not to blame him for what's happened; more to make the point that good practices (like those Steve suggests) put to use by people without understanding the full ramifications of that advice can lead to some nasty misconceptions.

Thinking about performance holistically, I'm immediately reminded of the talks John Resig did shortly after Sizzle was released and competitors started popping up everywhere to convince people that selector engines were no longer the performance bottleneck in Javascript. People had become obsessed with a single point of performance and were ignoring the "bigger fish."

So here we are in 2011, and people are still regurgitating the idea that "using CSS is faster than images" without truly understanding the implications of that statement (and how untrue it can be under the right circumstances).  An analogy that I like to use comes from the world of 3D gaming: Rendering something (for example, a drop shadow) in CSS and markup is the real-time, in-game rendering while downloading an appropriately compressed drop shadow png image is using a pre-rendered full-motion-video cut scene. Sure, the initial payload may be somewhat heavier; but again, looking at the complete picture, pre-rendering allows for overall higher performance.

I mention the drop shadow example because it's absolutely real.  When the team at Netflix was building the initial versions of our web-based UI for connected devices, we ran into serious performance problems on all sorts of devices like the PlayStation3. One of the things we found that significantly sped up the application was to avoid using CSS effects like shadows, gradients, and other "CSS effects;" instead, favoring pre-rendered effects. In some cases, the performance gains were nothing short of astonishing.

To the examples provided by the initial Hacker News article, let's look at the Twitter logo demonstration. It's 27 DOM elements and over 4 kilobytes of CSS.  So already we have a comparably sized payload, a more complex DOM, and now it's time to add flow and paint operations to actually do the rendering of the elements.  There's a lot of calculations happening to position and style the elements to appear as the Twitter logo.  By using an image, we bypass the calculations on layout and painting that need to be done to do the rendering; those calculations were already completed.

Performance is more than optimizing for the network layer and initial load-time of a page. As with anything, there are tradeoffs to be made (add a little download time, speed up the rendering) and in different situations, there will be different speed winners based on those tradeoffs. More interesting when working with lower-end devices (such as mobile devices) are the tradeoffs between memory use and computation cycles that I hope to cover in depth in future blog posts.

Sadly, there are few resources that cover the topic of in-browser performance in any more depth than a cursory level. I've only touched on it here; though I do hope to continue writing about it. That said, before we can even start talking intelligently about working on performance from a holistic approach rather than focusing on individual concerns, we need to learn to get past the dogmatic rules that are holding us back.

Related Reading: Pure CSS Icons: Make the Madness Stop