如何使用HTML和CSS实现一个简洁的弹出框布局

<div id="popup-container">
    <div class="popup-box">
        <h2 class="popup-title">这里是标题</h2>
        <div class="popup-content">
            这里是内容
        </div>
        <button class="popup-close">关闭</button>
    </div>
</div>

#popup-container {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.5);
    display: flex;
    justify-content: center;
    align-items: center;
}

.popup-box {
    background-color: #FFFFFF;
    border-radius: 5px;
    padding: 20px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.5);
    max-width: 400px;
    width: 80%;
}

.popup-title {
    font-size: 18px;
    color: #333333;
    margin: 0 0 10px;
}

.popup-content {
    font-size: 14px;
    color: #666666;
    margin-bottom: 20px;
}

.popup-close {
    background-color: #CCCCCC;
    border: none;
    color: #FFFFFF;
    padding: 10px 15px;
    border-radius: 3px;
    cursor: pointer;
}

.popup-close:hover {
    background-color: #999999;
}

#popup-container {
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s ease;
}

#popup-container.active {
    opacity: 1;
    pointer-events: auto;
}

.popup-box {
    transfORM: translate(-50%, -50%) scale(0.5);
    transition: transform 0.3s ease;
}

.popup-box.active {
    transform: translate(-50%, -50%) scale(1);
}

var popupContainer = document.getElementById("popup-container");
var popupClose = document.querySelector(".popup-close");

popupClose.addEventListener("click", function() {
    popupContainer.classList.remove("active");
});

function showPopup() {
    popupContainer.classList.add("active");
}

showPopup();