Last week, Stoyan Stefanov and I spoke at Velocity Conference about optimizing massive CSS. We talked about our experiences optimizing large-scale sites like Facebook and Yahoo!, and we discussed our findings regarding the CSS efficiency of the Alexa Top 1000 websites.
Velocity was kind enough to share videos of the session:
What is the state of the internet regarding CSS performance? Kind of sad. We aren’t getting a lot of the basics right, and when we look at the more advanced techniques, there are some spectacular examples of what-not-to-do. Why do we care about CSS performance? As Stoyan talks about in the beginning of the video, it blocks progressive rendering and it is very difficult to auto-minify.
These basic rules, made famous by YSlow, have been around for a long time, and yet our data showed that many sites in the Alexa Top 1000 are still not employing the most basic techniques.
- 42% Don’t GZIP CSS
- 44% Have more than 2 CSS external files
- 56% Serve CSS with cookies (yummy to eat, bad for static content)
- 62% Don’t minify (check out the YUI Compressor!)
- 21% Have greater than 100K of CSS
More Advanced Techniques
I talked about these more advanced tests for the first time at Velocity. Here is how the top 1000 sites stack up and some recommendations for reasonable results.
|Occurrences in one page|
|Declaration||Max||Percentage too many
|Percentage sites *way* too many
|float||733||56%||13%||If you have a good nestable grids system, you shouldn’t need many floats. The worst offender in the Alexa Top 1000 sites declared the float property more than 700 times! Aim for less than 10.|
|h1-6||511||56%||9%||There are only so many usable font sizes on the web. Below 10px in most fonts is legible only by mice and few sites use really large typography as a design element. Imagine that a site chooses to use 24px as their max. That leaves 14 different sizes, however, we need to divide that number by two because most users can’t see subtle differences like a 1px change in font size. That leaves seven different heading sizes, which means 56% of sites in the Alexa top 1000 have too many heading declarations.|
|margin: 0||518||64%||14%||Different browsers have different default stylesheets. These stylesheets define how elements should look if you haven’t chosen an alternate style. It is important to get all browsers to the same starting point because it eliminates bugs and time wasted testing simple browser compatibility issues. This is should be accomplished using a reset stylesheet such as the one included in YUI. When a reset stylesheet isn’t used, margin zero tends to be sprinkled throughout the stylesheet as developers try to cope with browser differences in the absense of an abstracted solution. Setting the default margins to zero is the most basic job of a reset stylesheet, which means that 64% of the Alexa Top 1000 sites could benefit from including reset.css.|
|padding: 0||510||62%||10%||Excessive declarations of padding zero are similar to margins (see the above description). The worse offender in this case declared padding zero 510 times.|
|font-size||889||78%||23%||Headings (h1-6) often get hidden in class names, which can disguise typography efficiency issues. It is helpful to grep "font-size" to get an idea how many hidden headings exist on the site. The same rules apply to font-size that were explained under h1-6, so in fact the problem is much worse than our initial estimate. These figures mean that 78% of the Alexa Top 1000 sites have excessive heading declarations when we consider hidden headings. In addition, 22% of sites may not be getting the SEO benefits of properly using heading elements.|
|!important||526||-||12%*||Important overrides all other declarations specificity, so it can be dangerous. If used correctly, only on leaf nodes, it can be a powerful tool for creating typography and spacing mixins that stand outside of the normal cascade. On the other hand, excessive use of important is a sure indication of specificity wars. Specificity wars are what happens when developers start trying to beat each others specificity, rather than having a real solid architecture and code standards. Eventually, like the worst offender in this case with 526 important properties, you can end up in a case where nearly every property is marked important. This means that 12% of the Alexa Top 1000 Sites probably has an internal specificity war in it’s web team.|
Happy perf optimizing!