css 背景图片适配

MDN详细的解释点这里

#all{
    background: url(img/boardBg.png);
    position: absolute;
    width:100%;
    min-height:auto;
}
@media screen and (max-height: 750px) {
    #all {
        min-height: 750px;
    }
}
@media screen and (min-height: 750px) {
    #all {
        min-height: 100vh;
    }
}