navDemo.css

* {
    margin: 0;
    padding: 0;
}

a {
    text-decoration: none;
}


body {
    height: 100vh;
    background: grey url(./wall.jpg) no-repeat center center;
    background-size: cover; /* 就算屏幕超级大都没问题 */
}
/*
    todo 内联元素居中，在父元素上添加text-align: center;
*/
main {
    text-align: center;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
}

.wrapper {
    display: inline-block;
    background: rgba(255,255,255, 0.2);
    border-radius: 10px;
}

#content > div:nth-child(2) {
    margin-left: -22px;
}

#content > div:nth-child(3) {
    margin-left: -90px;
}

/*
    todo 父元素releative 子元素absoluted
*/
kbd {
    border: 1px solid red;
    width: 50px;
    height: 40px;
    display: inline-block;
    text-transform: uppercase;
    position: relative;
}

kbd > button {
    position: absolute;
    right: 0;
    bottom: 0;
    display: none;
}

kbd:hover > button {
    display: inline-block;
}

.key {
    border-radius: 7px;
    border: 1px solid #373737;
    font-size: 16px;
    /*line-height: 40px;*/
    font-family: Helvatical;
    width: 50px;
    height: 40px;
    color: #C5C5C5;
    background: linear-gradient(to bottom, #292929 0%,#111111 100%);
    box-shadow: 0 0 0 1px #1A1B1C, 0 0 0 2px #1F2020, 0 2px 0 2px #1F2020;
    position: relative;;
}

.key .icon-img {
    width: 16px;
    height: 16px;
    position: absolute;
    left: 4px;
    bottom: 2px;
}

.key .text {
    position: absolute;
    left: 4px;
    top: 2px;
}

.out-div {
    margin: 16px;
}

.out-div .key {
    margin: 0 10px;
}