SOURCE

<div>
    <span>你知道吗？再不努力，</span>
    <br>
    <span>就废了</span>

div span{
    background: linear-gradient(90deg, red, blue) no-repeat right bottom;
    background-size: 0 3px;
    transition: background-size 1s;
    line-height: 2;
}
div span:hover{
    background-size: 100% 3px;
    background-position-x:left;
}