js 弹窗al

window.onload = function() {
    alert('页面加载完成！');
};

<!-- HTML -->
<div id="myModal" class="modal">
    <div class="modal-content">
        <span class="close">&times;</span>
        <p>这是一个自定义弹窗！</p>
    </div>
</div>

<!-- CSS -->
<style>
.modal {
    display: none; 
    position: fixed; 
    z-index: 1; 
    left: 0;
    top: 0;
    width: 100%; 
    height: 100%;
    overflow: auto; 
    background-color: rgb(0,0,0); 
    background-color: rgba(0,0,0,0.4); 
}

.modal-content {
    background-color: #fefefe;
    margin: 15% auto; 
    padding: 20px;
    border: 1px solid #888;
    width: 80%; 
}

.close {
    color: #aaa;
    float: right;
    font-size: 28px;
    font-weight: bold;
}

.close:hover,
.close:focus {
    color: black;
    text-decoration: none;
    cursor: pointer;
}
</style>

<!-- JavaScript -->
<script>
var modal = document.getElementById("myModal");
var span = document.getElementsByClassName("close")[0];

span.onclick = function() {
    modal.style.display = "none";
}

window.onclick = function(event) {
    if (event.target == modal) {
        modal.style.display = "none";
    }
}

// 显示弹窗
modal.style.display = "block";
</script>

/* 添加到CSS中 */
@media only screen and (max-width: 600px) {
    .modal-content {
        width: 90%; 
        margin: 30% auto; 
    }
}