x-ray is a visual debugger for your HTML, executable via a bookmark!

x-ray.js

javascript: (
    function () {
        var elements = document.body.getElementsByTagName("*");
        var items = [];
        for (var i = 0; i < elements.length; i++) {
            if (elements[i].innerHTML.indexOf("background:#000!important;color:#0f0!important;outline:solid #f00 1px!important;") != -1) {
                items.push(elements[i]);
            }
        }

        if (items.length > 0) {
            for (var i = 0; i < items.length; i++) {
                items[i].innerHTML = "";
            }
        } else {
            document.body.innerHTML += "<style>*{background:#000!important;color:#0f0!important;outline:solid #f00 1px!important;}</style>";
        }
    }
)();