DOMContentLoaded和load
DOMContentLoaded事件先执行,load事件后执行。
document.addEventListener('DOMContentLoaded', function () {
    console.log(1)
});
window.onload = function () {
    console.log(2)
}
document.addEventListener('DOMContentLoaded', function () {
    console.log(1)
});
window.onload = function () {
    console.log(2)
}
