Responsive text by JavaScript

I'm working on a JavaScript plugin that makes font-sizes flexible (for a responsive layout). The code works well so far, but I'm sure that it's capable of improvement (especially if you look at the global variable).

(function () {
    var extend = function (obj, ext) {
        for (var key in ext) {
            if (ext.hasOwnProperty(key)) {
                obj[key] = ext[key];
            }
        }
        return obj;
    };

    window.txt = function (el, k, c) {
        var s = extend({
            n: Number.NEGATIVE_INFINITY,
            x: Number.POSITIVE_INFINITY
        }, c);

        var fit = function () {
            var a = k;

            var i = function () {
                el.style.fontSize = Math.max(Math.min(el.clientWidth / a, parseFloat(s.x)), parseFloat(s.n)) + "px";
            };
            i();
            window.addEventListener("resize", i);
            window.addEventListener("orientationchange", i);
        };

        if (el.length) {
            for (var i = 0; i < el.length; i++) {
                fit(el[i]);
            }
        } else {
            fit(el);
        }

        return el;
    };
})();

DEMO

Meaning of the used variables:

• n = minFontSize
• x = maxFontSize
• s = settings
• c = config
• el = element
• i → running the function
• a or k = fontRatio/compressor