parsecss

Convert CSS Stylesheets into a simple JSON structure.

parsecss was written to support critical CSS inlining on dynamic pages. The JSON structure it produces is optimized for figuring out which CSS rules should be applied to a given page. We use postcss to do most of the heavy lifting, but our JSON output makes it easier to use than an AST for certain use cases.

Installation

npm install -g parsecss

Usage

To use parsecss you simply pass it the contents of a CSS stylesheet (or path).

parsecss -f path/to/file.css

// via stdin
cat path/to/file.css | parsecss

You can also use parsecss programatically in node:

var parseCSS = require('parsecss');
var output = parseCSS(css);

JSON Structure

parsecss was written to support critical CSS inlining on dynamic pages. As such, the JSON structure it produces is optimized for figuring out which CSS rules should be applied to a given page.

Terminology:

1. Rules - A CSS rule refers to a selector + its declarations.
2. Selectors - For each rule, the selectors define which elements are affected.

We consider each rule in a @media query as its own rule to ensure that we don't miss out on any rules that should be included.

Global Rules

We consider all rules with selectors that don't contain class names to be global. Since we don't use IDs in our CSS selectors, this effectively means that all rules that directly affect tags (eg. html, body, a) are global.

Class List Pairs

For each non global rule, we parse the class names that are contained in it (accounting for pseudo selectors).

Eg. .home .link.a {} => ['home', 'link']

@font-face Rules

These rules are split into their own section since they are almost always critical and are required to fetch the fonts as soon as possible.

Keyframes

Keyframe rules are parsed so that they are keyed by their names to allow for conditional inclusion of certain critical keyframes on the page.

Sample Output

Each rule in the output is minified using cssmin.

{
    "globalCss": ["footer{display:none}", "a{text-decoration:underline}"],
    "fontfaceCss": ["@font-face{...}"],
    "keyframesCss":[
        ["fadeOut","@keyframes fadeOut{from{opacity:1}to{opacity:0}}"],
        ["fadeInUp","@-webkit-keyframes fadeInUp{}"]
    ],
    "classListCssPairs": [
        [["Button","Topic"],".Button.Topic{border:0}"],
        [["NavHeader","link"],"@media screen and (max-width:434px){.NavHeader .link{...}}"],
    ]
}