It’s not as simple as just calling WebKits getMatchedCSSRules(), it does return the matched rules in order of priority (altho I’ve seen no mention of this order in the docs), but the order does not take regard to property important priority and does not include element styles. So I ended up with this function:

getMatchedStyle

function getMatchedStyle(elem, property){
    // element property has highest priority
    var val = elem.style.getPropertyValue(property);

    // if it's important, we are done
    if(elem.style.getPropertyPriority(property))
        return val;

    // get matched rules
    var rules = getMatchedCSSRules(elem);

    // iterate the rules backwards
    // rules are ordered by priority, highest last
    for(var i = rules.length; i --> 0;){
        var r = rules[i];

        var important = r.style.getPropertyPriority(property);

        // if set, only reset if important
        if(val == null || important){
            val = r.style.getPropertyValue(property);

            // done if important
            if(important)
                break;
        }
    }

    return val;
}

Example

Given the following code and style rules:

<div class="b">div 1</div>
<div id="a" class="a d2">div 2</div>
<div id="b" class="b d3" style="width: 333px;">div 3</div>
<div id="c" class="c d4" style="width: 44em;">div 4</div>

<style>
div      { width: 100px; }
.d3      { width: auto !important; }
div#b    { width: 80%;   }
div#c.c  { width: 444px; }
x, div.a { width: 50%;   }
.a       { width: 75%;   }
</style>

this JS code

var d = document.querySelectorAll('div');

for(var i = 0; i < d.length; ++i){
    console.log("div " + (i+1) + ":  " + getMatchedStyle(d[i], 'width'));
}

gives the following widths for the divs:

div 1:  100px
div 2:  50%
div 3:  auto
div 4:  44em

(At jsFiddle)