mincss

mincss is an extendible CSS minifier written in OCaml. It contains a complete parser for the CSS3 language, along with consistent type definitions and a traversal utility function for use in transformation passes.

mincss is currently still in development, finished components are the parser, stringification (along with whitespace compression), and color compression. Rulset merging is partially documented below but currently unimplemented.

Features

• Whitespace compression
• Color compression
• Creation of shorthand properties (e.g. font and background)
• Ruleset merging (see below)
• Command-line interface and web interface

Ruleset merging

Apart from simply writing the CSS in a shorter format (i.e. whitespace/color/shorthand compression), mincss attempts to restructure rule sets such that the resulting stylesheet is the minimal representation of the input. For example:

a { color: red }
p { color: red }

can be written much shorter as:

a, p { color: red }

Merging selectors is something that is done by the programmer in most cases, but the example above may occur when multiple different CSS files are merged into one, or when a large CSS file is structured in such a way that the definitions of a and p are far apart. Another thing that may happen is that some framework file defines a default style for a selector, that is then overwritten in a custom stylesheet, for example:

/* file: framework.css */
a {
    border: 1px solid red;
}

...

/* file: my-special-page.css */
a {
    border-color: blue;
}

which can be merged into:

a {
    border: 1px solid blue;
}

Compression phases

To achieve the features listed above, the input stylesheet is rewritten in a number of steps:

1. Parse the input CSS, producing an Abstract Syntax Tree (AST) in accordance with the CSS syntax definition. This eliminates unnecessary whitespaces and catches syntax errors.
2. Transform shorthand declarations into a separate declaration for each expression in the shorthand.
3. Duplicate rulesets for each of its selectors, so that every ruleset has exactly one selector.
4. Create a new declaration block for each property declaration on each selector, e.g. a,p{color:red;border:blue} becomes a{color:red} a{border:blue} p{color:red} p{border:blue}.
5. Prune duplicate declarations (when the same property is overwritten for the same selector). Note: this may be disabled with --no-prune for stylesheets that contain duplication hacks.
6. Combine selectors for identical blocks (as long as the correct order of declarations is preserved).
7. Concatenate blocks for identical (combinations of) selectors.
8. Optimize individual declaration blocks by generating shorthand declarations and simple compression on expressions (i.e. color compression, removal of unnecessary string quotes, etc.).
9. Output the resulting AST with minimal whitespace.

Building mincss

TODO

Compression scores

TODO: compare to existing minifiers