index.css

html,
body {
    overflow: hidden;
    height: 100%;
    background-color: black;
}

/* 其他样式保持不变 */
@media screen and (max-width: 600px) {}

@keyframes fade {
    0% {
        opacity: 0;
    }

    50% {
        opacity: 1;
    }

    70% {
        opacity: 1;
    }

    100% {
        opacity: 0;
        z-index: -1;
    }
}

@keyframes appear {
    0% {
        opacity: 0;
    }

    80% {
        opacity: 0;
    }

    100% {
        opacity: 1;
    }
}

a {
    text-decoration: none;
    color: #333;
}

#my-image {
    width: 50vw;
    display: block;
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    animation: fade 2s linear forwards;
}

.button-area {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100%;
    width: 100%;
}

.button {
    list-style-type: none;
    text-align: center;
    text-decoration: none;
    font-size: 6vh;
    color: #333;
    opacity: 0.8;
    height: 10vh;
    width: 100vw;
    padding: 0;
    border: none;
    margin: 0;
    display: inline-block;
    cursor: pointer;
    background-color: lightgray;
}

.button:hover {
    opacity: 0.6;
}

.navtog {
    position: absolute;
    top: 0;
    z-index: 2;
    height: 30px;
    width: 100%;
    background-color: black;
    background-image: url("arrow_button.png");
    background-size: cover;
    /* 调整背景大小适应按钮宽度 */
    background-position: center;
    /* 设置背景图片居中 */
    background-repeat: no-repeat;
    text-align: center;
    font-size: 50px;
    cursor: pointer;
    opacity: 1;
    animation: appear 2s linear;
}

.navtog:active {
    color: red;
}

.dropdown {
    padding: 0;
    margin: 0;
    position: absolute;
    top: 0;
    width: 100%;
    height: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    overflow: hidden;
    transition: height 0.3s ease;
    z-index: 1;
}

.dropdown .button:hover {
    background-color: #999;
    opacity: 0.8;
}

.navtog.active+.dropdown {
    height: 60%;
}

iframe {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 30px;
    opacity: 1;
    animation: appear 2.5s linear;
    border: none;
}