There are quite a few CSS minifiers out there, which can bring the raw size of your CSS files down substantially. Very few of them, however, will also optimise your CSS for gzip compression. To that end, I’ve written a small Java application that will read in a CSS file and output its contents to stdout or another file in a format that’s optimised for gzipping.

The problem

A file will be most compressed when there are many recurring strings in the file. This means that when writing CSS files, this code:

.pony {
border: solid red 1px;
font-weight: bold;
}
.lemur {
border: solid red 1px;
font-weight: normal;
}

will be better-compressed than this:

.pony {
border: solid red 1px;
font-weight: bold;
}
.lemur {
font-weight: normal;
border: red solid 1px;
}

What it does

Specifically, my CSS minifier will perform the following transformations on the file:

• All comments are removed.
• The properties within all selectors are ordered alphabetically.
• The values for all properties are ordered alphabetically.
• All unnecessary whitespace is removed.

By way of example, the following CSS snippet:

body {
padding: 8px;
margin: 0;
background-color: blue;
color: white;
font-family: "Trebuchet MS", sans-serif;
}

h1 {
margin: 0;
padding: 0;
font-size: 200%;
color: #0F0;
font-weight: bold;
}

p {
margin: 0 0 2em;
line-height: 2em;
}

would be formatted to:

body{background-color:blue;color:white;font-family:"Trebuchet MS",sans-serif;margin:0;padding:8px;}h1{color:#0F0;font-size:200%;font-weight:bold;margin:0;padding:0;}p{line-height:2em;margin:0 0 2em;}

Compression results

A (very) large real-world CSS file of 78.2KB has a minified size of 63.7KB, which is a size decrease of nearly 20%. GZipping these two files, the un-minified file is reduced to 13.2KB, whilst the minified file weighs in at just 11.5KB — nearly 13% smaller.

Clearly, removing unnecessary whitespace and reordering the contents of the CSS selectors can greatly improve the compression of your CSS files, and thus make your pages deliver more quickly.

Comparison with YUI CSS Compression

As you can see in the chart below, the YUI compressor manages to make the original file around 1KB smaller than my minifier. However, when the two minified files are gzipped, my compression results in a file that is 11.5KB in size, whilst YUI only reduces down to 11.8KB.

Download

The source for this applet is available freely, and is a single Java file. It’s not particularly pretty, because I knocked this together in about half an hour.
Download
License: Mozilla Public License (In other words, do whatever you like with it. Credit is greatly appreciated, but not required.

Usage

First, if you haven’t done so yet, compile the code:

# javac CSSMin.java

Then, you can call the minifier by running

# java CSSMin in.css [out.css]

If you do not specify an output file, the resultant CSS will be printed to stdout (and can then be redirected as you wish).

Contact

If you have any questions or comments about this app, or if you find a bug or some weird behaviour, just comment on this post, and I’ll see what I can do.

If you find this utility useful, let me know! If you want to extend it, you’re free to do whatever you like with it, as it’s under the MPL. It would be nice, though, if you dropped me a line to let me know what you’re doing with it.

Future work

• Replace “0[px,em,%,etc]” with “0″.
• Replace “0 [0 [0 [0]]]” with “0″.
• Replace “0.6″ with “.6″ wherever possible.
• Replace all “rgb(64, 64, 64)” with “#404040″.
• Shorten colours from #112233 to #123 wherever possible.
• Remove all empty rules.
• Ordering of property valus according to the CSS spec, not just alphabetically.
• A cleaner interface, including the ability to stream and process from stdin.