Front-end performance is also a critical priority for Zen. That’s why Zen 5’s markup and CSS footprint is so small. After turning on Sass production-mode compilation, the total CSS is only 14k; this includes the layout, vertical rhythm support, print styles, all the appropriate vender prefixes (like -o and -khtml), CSS3-prettified tab styling and the 3 menu images that are included via Data URIs into the CSS. Zen includes no images and only 2 JavaScripts (which are only loaded conditionally by IE8 and lower, but also minified and pre-aggregated together).
Zen 5 also minimizes the number of HTTP requests to just one CSS file by always using the "all" media type in its .info file and writing CSS3 media queries inside the CSS files.