function countElements() {
    var elements = document.getElementsByTagName("li");
    alert(elements.length);
}

function doubleElements() {
    var elements = document.getElementsByTagName("li");
    var length = elements.length;
    var list = document.getElementById("list");
    var element = "<li>liタグだよ</li>";
    list.innerHTML = "";
    for(var i = 0; i < length * 2; i++) {
        list.innerHTML += element;
    }
}