<div id="dn">我是要显示或隐藏的元素</div>

<script>
    function handleWindowResize() {
        var element = document.getElementById('dn');
        var windowWidth = window.innerWidth;

        if (windowWidth > 900) {
            element.style.display = 'block';  // 显示元素
        } else {
            element.style.display = 'none';   // 隐藏元素
        }
    }

    // 初始加载时和窗口尺寸改变时触发判断
    window.addEventListener('load', handleWindowResize);
    window.addEventListener('resize', handleWindowResize);
</script>