xiaoben-as2-php/css/teaching/physics/base.css

.fl {
    float: left
}

.fr {
    float: right
}

.tal {
    text-align: left;
}

.tar {
    text-align: right;
}

.tac {
    text-align: center
}

.clearfix:after {
    content: ".";
    display: block;
    height: 0;
    clear: both;
    visibility: hidden;
}

.clearfix {
    *zoom: 1;
}