I've recently released version 1.4 of CCSS, a preprocessor/pretty-printer for CSS (Cascading Style Sheets). Though CCSS most definitely falls into the category of software developed to scratch a personal itch, it has gained other users in the meantime. Consequently, I reckon a few words are in order concerning the features/idiosyncrasies of this tool.

For those not familiar with CCSS or CSS preprocessors in general, the basic rationale for these tools is to fill some glaring shortcomings in the vanilla CSS language, namely the lack of variables and basic arithmetic operations. Many tools go further still, providing new ways to structure and organise CSS declarations.

There is certainly no lack of CSS preprocessors available, most of which have large user bases and plenty of fancy features. So, why CCSS? Does the world really need yet another CSS preprocessing tool? I cannot speak for the world. I can, however, relate that some years ago I was in need of a CSS preprocessor that supported variables, arithmetic, and did not choke on the few CSS3 constructs I was introducing into my stylesheets. Moreover, I was looking for a nice, clean syntax that felt like a natural extension to vanilla CSS. Finally, I don't think it was unreasonable to expect that a tool performing such a simple task to be fast. Unfortunately, the tools I tried back then did not quite meet these requirements, particularly where speed was concerned.

OCaml hackers being who they are, I reckoned that a better tool for my needs could be written in a weekend. And that's how CCSS was born. Fortunately, I was about right concerning the time estimate, which just goes to show how awesome the OCaml language and assorted tools (namely Menhir and Ulex) are for writing compilers.

This little history brings us to the most important point about CCSS that potential users should be aware of: it was never meant to be a fully-compliant superset of vanilla CSS. To illustrate this point, consider the humble semicolon, which according to the CSS spec is a declaration separator, and optional after the last declaration in a block. Omitting the last semicolon is, however, a brittle practice (it's all too easy to forget to add the semicolon when a new declaration is appended to the block), which is why users are advised to always write a semicolon at the end of each declaration, effectively treating it as a declaration terminator. Guess what? The CCSS grammar treats the semicolon as a mandatory terminator — spec be damned.

Variables are perhaps the most useful extension to CSS, and CCSS does support them. In previous versions, variables could only be assigned to expressions. The recently released version 1.4, however, introduces the possibility of assigning a whole declaration block to a variable. This feature is similar to what other preprocessors term mixins, so this is the nomeclature I'm using too.

The example shown below illustrates the use of variables to declare commonly used expressions. Note that syntactically, variable identifiers are distinguished by starting with a mandatory uppercase letter. This was chosen to make tooling (such as syntax highlighting in editors) easier.

Foo: 20em;
Bar: 1px solid black;
width: Foo;
border: Bar;

The second example concerns the use of variables to declare mixins, ie, declaration blocks that can be included within subsequent declaration blocks:

color: #fff;
background: #000;
font-weight: bold;

CCSS also extends CSS expressions with basic arithmetic operations (addition, subtraction, multiplication, and division). The operands must be CSS quantities (either dimensionless or with an attached unit), or other expressions that ultimately resolve into a quantity. Moreover, variables whose value is a quantity (or an expression which resolves into a quantity) may also be used as operand.

The operators are '+', '-', '*', and '÷'. Note that multiplication and division have precedence over addition and subtraction, but you may use parentheses to group operations. In addition, the choice of the non-ASCII character '÷' as division operator betrays CCSS's origins as tool designed to scratch a personal itch: it can be input with just a few easy-to-remember keystrokes in VIM, the editor I use. I had thus no motivation to find an ugly multi-character token to represent it. Consider thus the following input:

Left: 10em;
Right: 5em;
Total: Left + Right;
padding: (1.5em + 0.5em) * 2;
width: 2 * Total;

CCSS will produce the following output:

padding: 4em;
width: 30em;

The reader will have noticed that CCSS must be unit-aware when performing arithmetic. As a matter of fact, the programme performs a basic sanity check of units, and will complain if you try, for example, to add "1em" with "10px". By default, CCSS will not make any attempt to convert units even if they are convertible, such "cm" and "mm". If you wish for CCSS to attempt unit conversion, please provide option "--convert" on the command line (short version "-c").

Units can be grouped into four categories, and conversion is possible if the units belong to the same category. Upon conversion, the result will be the same unit as the first operand. The categories and corresponding units are as follows:

  • length: mm, cm, in, pt, pc
  • angle: deg, grad, rad
  • time: ms, s
  • frequency: hz, khz

As an illustration of unit conversion, the result for all the following arithmetic operations is the same, "2in":

foo1: 1in + 1in;
foo2: 1in + 2.54cm;
foo3: 1in + 25.4mm;
foo4: 1in + 72pt;
foo5: 1in + 6pc;

And that's it! Note that the project's development has recently moved to GitHub, so fork away!