CSS minifier and alphabetiser

Update: This project is now hosted on GitHub: https://github.com/barryvan/CSSMin/

There are quite a few CSS minifiers out there, which can bring the raw size of your CSS files down substantially. There are, however, significant gains to be made if the CSS is minified so that it gzips better. 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 gzipped file will be stored most efficiently 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;
}

In the first sample, notice that we have a very long string that occurs twice:

 {
border-style: solid red 1px;
font-weight: 

In the second sample, there are strings that occur more than once, but they’re much shorter. The gzip algorithm can, in the first case, replace that entire long string with a much shorter placeholder.

What it does

So, how can we optimise CSS for gzipping, then? A file that’s minified using this CSS Minifier will have these operations applied:

  • All comments removed.
  • The properties within all selectors ordered alphabetically.
  • The values for all properties ordered alphabetically.
  • All unnecessary whitespace removed.
  • Font weights replaced by their numeric counterparts (which are shorter).
  • Quotes stripped wherever possible.
  • As much text as possible transformed to lowercase.
  • Prefixed properties (for example, -moz-box-sizing) placed before the unprefixed variant (box-sizing).
  • Colours simplified from rgb() to six- or three-digit hex values, or simple names.
  • Units on values of 0 stripped.
  • Multi-parameter items simplified to as few parameters as possible.
  • Various other small tweaks and adjustments made.

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 the following (note that line breaks have been added for legibility — no line breaks appear in the final output):

body{background-color:blue;color:#fff;font-family:"trebuchet ms",sans-serif;
margin:0;padding:8px}h1{color:#0f0;font-size:200%;font-weight:700;margin:0;
padding:0}p{line-height:2em;margin:0 0 2em}

Compression results

These are the results of compressing the main CSS file for one of the webapps I develop at work.

  Original size (bytes) Gzipped size (bytes)
Plain 81938 12291
YUI 64434 10198
LotteryPost 63609 10165
CSS Drive 69275 10795
CSSMin 63791 9896

Download

Head over to GitHub to download the source.

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.

You can also raise issues on GitHub, fork the project, commit changes, and more.

If you find this utility useful, let me know!