<script>
    window.onload = function() {
        document.onmousemove = function(event) {
            var spanEl = document.getElementsByTagName('span')[0];
            var text = "X: " + event.pageX + ", Y: " + event.pageY;
            spanEl.innerHTML = text;
        }
    }
<script>

帮题主调了一下缩进，不然看着好蛋疼。。。
题主说的用 JavaScript 写应该意思是原生 JS 吧，其中用到的几个方法:

• 获取节点 - getElementById getElementsByClassName getElementsByTagName querySelector 等

• 监听事件 - addEventListener

• dom 加载完毕 - readystatechange 或者 DOMContentLoaded

• 获取坐标 - 鼠标事件的事件对象

• 修改节点文本 - textContent

题主可以先自己尝试一下。

<script>
    window.onload = function() {
        document.onmousemove = function(event) {
            var spa = document.getElementsByTagName('span')[0];
            var text = "X: " + event.pageX + ", Y: " + event.pageY;
            spa.innerHTML = text;
        }
    }
<script>

<script>
    var ready = (function () {
            var readyState = false;
            return function (callback) {
                if (readyState) {
                    callback();
                }
                if (window.addEventListener) {
                    window.addEventListener('DOMContentLoaded', function () {
                        readyState = true;
                        callback();
                    }, false);
                } else {
                    var complete = function () {
                        if (document.readystate == 'interactive' || document.readystate == 'complete') {
                            readyState = true;
                            callback();
                            clearInterval(timer)
                        }
                    };
                    var timer = setInterval(function () {
                        try {
                            document.documentElement.doScroll('left');
                            clearInterval(timer);
                            readyState = true;
                            document.detach('readystatechange', complete);
                            callback();
                        }
                        catch (ex) {
                        }
                    }, 50);
                    document.attachEvent('readystatechange', complete);
                }
            }
        })();
        ready(function () {
            // 获取span没有必要放到onmousemove里面.如果放到里面每次触发鼠标事件都会获取元素,完全没有那个必要,只获取一次即可.
            var spa = document.getElementsByTagName('span')[0];
            document.onmousemove = function(event) {
                spa.innerHTML = "X: " + event.pageX + ", Y: " + event.pageY;
            }
        })
<script>