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rCSSmin - A CSS Minifier For Python


  1. Introduction
  2. Copyright and License
  3. System Requirements
  4. Installation
  5. Documentation
  6. Bugs
  7. Author Information


RCSSmin is a CSS minifier written in python.

The minifier is based on the semantics of the YUI compressor, which itself is based on the rule list by Isaac Schlueter.

This module is a re-implementation aiming for speed instead of maximum compression, so it can be used at runtime (rather than during a preprocessing step). RCSSmin does syntactical compression only (removing spaces, comments and possibly semicolons). It does not provide semantic compression (like removing empty blocks, collapsing redundant properties etc). It does, however, support various CSS hacks (by keeping them working as intended).

Here's a feature list:

  • Strings are kept, except that escaped newlines are stripped

  • Space/Comments before the very end or before various characters are stripped: :{});=>],! (The colon (:) is a special case, a single space is kept if it's outside a ruleset.)

  • Space/Comments at the very beginning or after various characters are stripped: {}(=:>[,!

  • Optional space after unicode escapes is kept, resp. replaced by a simple space

  • whitespaces inside url() definitions are stripped, except if it's a quoted non-base64 data url

  • Comments starting with an exclamation mark (!) can be kept optionally.

  • All other comments and/or whitespace characters are replaced by a single space.

  • Multiple consecutive semicolons are reduced to one

  • The last semicolon within a ruleset is stripped

  • CSS Hacks supported:

    • IE7 hack (>/**/)
    • Mac-IE5 hack (/*\*/.../**/)
    • The boxmodelhack is supported naturally because it relies on valid CSS2 strings
    • Between :first-line and the following comma or curly brace a space is inserted. (apparently it's needed for IE6)
    • Same for :first-letter

rcssmin.c is a reimplementation of in C and improves runtime up to factor 100 or so (depending on the input). docs/BENCHMARKS in the source distribution contains the details.

.. _YUI compressor:

.. _the rule list by Isaac Schlueter:


Copyright 2011 - 2021 André Malo or his licensors, as applicable.

The whole package (except for the files in the bench/ directory) is distributed under the Apache License Version 2.0. You'll find a copy in the root directory of the distribution or online at:


Supported python versions are 2.7 and 3.6+.

You also need a build environment for python C extensions (i.e. a compiler and the python development files).


Using pip

$ pip install rcssmin

Using distutils

Download the package, unpack it, change into the directory

$ python install

The command above will install a new "rcssmin" package into python's library path.


rCSSmin effectively consists of two files: and rcssmin.c, the latter being entirely optional. So, for simple integration you can just copy into your project and use it.


The module provides a simple function, called cssmin which takes the CSS as a string and returns the minified CSS as a string.

The module additionally provides a "streamy" interface:

$ python -mrcssmin <css >minified

It takes two options:

-b Keep bang-comments (Comments starting with an exclamation mark) -p Force using the python implementation (not the C implementation)

The latest documentation is also available online at


No bugs, of course. ;-) But if you've found one or have an idea how to improve rcssmin, feel free to send a pull request on github or send a mail to


André "nd" Malo, GPG: 0x029C942244325167

If God intended people to be naked, they would be born that way. -- Oscar Wilde