动画模态窗口始终保持CSS垂直居中

Question

我在动画Modal Windows上遵循了此w3schools教程。现在，我有一个模态窗口，该窗口从顶部滑入并包含一个图像。当我将窗口缩放到比其高时更宽时，图像将随之缩放，并且“模式窗口”的底部从屏幕底部掉落。有没有一种方法可以使窗口动画化，因此无论视口高度如何，窗口始终停在屏幕的中央；第二，将Modal的底部锁定在距视口底部一定的高度吗？

这是我的HTML（JavaScript用图像填充.container）：

<div id="modal" class="modal">
<div class="modal-content">

    <div id="modal-header" class="modal-header">
    <span class="close">&times;</span>
    <h2>Modal Header </h2>
        <h3> sub </h3>
    </div>

    <div class="modal-body">
        <div class="container">

        </div>
    </div>
</div>

这是我的CSS：

/* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    padding-top: 15vh; /* Location of the box */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    max-height: 100%;
    overflow: hidden;
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.65); /* Black w/ opacity */
}

/* Modal Content */
.modal-content {
    position: relative;
    background-color: #262626;
    margin: 0 auto;
    border: 1px solid #888;
    width: 95%;
    box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2),0 6px 20px 0 rgba(0,0,0,0.19);
    -webkit-animation-name: animatetop;
    -webkit-animation-duration: 0.4s;
    animation-name: animatetop;
    animation-duration: 0.4s
}

.modal-header {
    font-size: 10px;
    text-overflow: scale;
    padding: 2px 16px;
    background-color: black;
    color: white;
}

@-webkit-keyframes animatetop {
    from {top:-300px; opacity:0}
    to {top:0; opacity:1}
}

@keyframes animatetop {
    from {top:-300px; opacity:0}
    to {top:0; opacity:1}
}

.modal-body {
    padding: 1px 6px;
}

.container img {
    width: 100%;
    height: auto;
}