CSS Diff

As a proof of concept, we can do a small trick here and avoid creating extension. Chrome keeps elements you select via 'inspect element' button in variables. Last selected element in $0, one before that in $1 etc. Basing on this, I've created a small snippet that compares last two inspected elements:

(function(a,b){    
    var aComputed = getComputedStyle(a);
    var bComputed = getComputedStyle(b);

    console.log('------------------------------------------');
    console.log('You are comparing: ');
    console.log('A:', a);
    console.log('B:', b);
    console.log('------------------------------------------');

    for(var aname in aComputed) {
        var avalue = aComputed[aname];
        var bvalue = bComputed[aname];

        if( aname === 'length' || aname === 'cssText' || typeof avalue === "function" ) {
            continue;
        }

        if( avalue !== bvalue ) {
            console.warn('Attribute ' + aname + ' differs: ');
            console.log('A:', avalue);
            console.log('B:', bvalue);
        }
    }

    console.log('------------------------------------------');
})($0,$1);

How to use it?

Inspect two elements you want to compare, one after another, and paste the code above to console.

Result