Order CSS based on Selector Specificity

Question

I have parsed a given CSS file/string into a JSON object like so:

{
    "#header": {
        "color": "#000000"
    },
    "#header h1": {
        "color": "#000"
    },
    "h1": {
        "color": "#4fb6e5"
    }
}

What I want to do now is re-order them based on Specificty. In this case, the #header h1 should come after the h1 in the JSON object as this is how they'll be applied in the browser.

How can I do this? Are there any existing libraries for this? Or any useful libraries to help with this?

I can use both Javascript/jQuery or PHP to do this. I'm looking for implementation advice and hopefully this has already been done!

Answer 1

Short answers:

Are there any existing libraries for this?

No, not one I'm aware of that does this "out-of-the-box" for you.

Or any useful libraries to help with this?

Yes, there is json_decode and uksort:

$specificity = function($selector) {

    /*
     * @link http://www.w3.org/TR/selectors/#specificity
     *
     * 9. Calculating a selector's specificity
     *
     * A selector's specificity is calculated as follows:
     * 
     *  * count the number of ID selectors in the selector (= a)
     *  * count the number of class selectors, attributes selectors, and 
     *    pseudo-classes in the selector (= b)
     *  * count the number of type selectors and pseudo-elements in the 
     *    selector (= c)
     *  * ignore the universal selector
     *
     * Selectors inside the negation pseudo-class are counted like any other, 
     * but the negation itself does not count as a pseudo-class.
     *
     * Concatenating the three numbers a-b-c (in a number system with a large 
     * base) gives the specificity.
     */
    ...
    return (int) $result;
}

$compare = function($a, $b) use ($specificity) {
    return $specificity($a) - $specificity($b)
};

$array = json_decode('{"yours" : "json"}', true);

uksort($array, $compare);

echo json_encode((object) $array);

As this code example shows, it only explains how to calculate the specificity in a comment and it does not contain the code. That's just because I don't have that code at hand, but I have put in there the specification how this is done (at least for CSS 3).

If you're looking for a CSS selector parser, I know about XDOM (because I wrote it). It's available on github: https://github.com/hakre/XDOM - It's a 100% CSS 3 compatible CSS selector parser.

To my best knowledge that is most of what you get as of today in terms of ready-made solutions. All other CSS selector parsers I know of are not compatible with the CSS 3 standard in full because they don't follow the W3C specification. Which might be good for you: If you don't need strict CSS3 compatbility, you might find some other code-chunks that suit your needs already.