А вот так:

function getViewportSize() {
    var size = {};

    if (typeof window.innerWidth != 'undefined') {
        size.width  = window.innerWidth,
        size.height = window.innerHeight
    }
    else if (typeof document.documentElement != 'undefined'
        && typeof document.documentElement.clientWidth !=
        'undefined' && document.documentElement.clientWidth != 0) {
            size.width  = document.documentElement.clientWidth,
            size.height = document.documentElement.clientHeight
    } else {
        size.width  = document.getElementsByTagName('body')[0].clientWidth,
        size.height = document.getElementsByTagName('body')[0].clientHeight
    }

    return size;
}

В Mootools лучше делать так:

window.implement({
    getViewportSize: function() {
        var size = {};

        if (typeof window.innerWidth != 'undefined') {
            size.width  = window.innerWidth,
            size.height = window.innerHeight
        }
        else if (typeof document.documentElement != 'undefined'
            && typeof document.documentElement.clientWidth !=
            'undefined' && document.documentElement.clientWidth != 0) {
                size.width  = document.documentElement.clientWidth,
                size.height = document.documentElement.clientHeight
        } else {
            size.width  = document.getElementsByTagName('body')[0].clientWidth,
            size.height = document.getElementsByTagName('body')[0].clientHeight
        }

        return size;
    }
});

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