body {
    width: 800px;
    margin-left: 300px;
    margin-left: 300px;
}
.header {
    height: 40px;
    margin-top:2px;
    margin-bottom: 2px;
}

.header>a{
    height: 40px;
    line-height: 40px;
    font-size: 36px;
}
.introduction {
    font-size: 28px;
    margin-top: 300px;
}