/*等同如下*/
transition-property: width;
transition-duration: 1s;
transition-timing-function: linear;
transition-delay: 2s;

/*css代码*/

h2{
    position: relative;
    padding: 15px;
    text-align: center;    
}
button{
    width: 100px;
    height: 40px;
    border-radius: 15px;
    border: none;
    background: #188FF7;
    color: #fff;
    outline: none;
    cursor: pointer;
    font-weight: bold;
}
button:hover{
    background: #188EA7;
}
.container{
    width: 600px;
    display: flex;
    flex-direction: column;
    align-items: center;
    margin: 0 auto;
    
}
.line{
    position: absolute;
    left: 0;
    bottom: 0;
    height: 3px;
    width: 100%;
    transition: transform .5s;
    background: #188EA7;
    color: #188EA7;
    transform: scaleX(0);
    z-index: 1111;            
}

@keyframes changeColor1{
    from{
        color: #000;
    }
    to{
        color: #188EA7;
    }
}
@keyframes changeColor2{
    from{                
        color: #188EA7;
    }
    to{
        color: #000;
    }
}
<!--html代码-->

<div class="container">
    <h2 id="title">
        百度前端学院
        <i class="line" id="line"></i>
    </h2>
    <button id="change">Change</button>
</div>
//js部分代码

(function () {
    let btn = document.getElementById('change');
    let h2 = document.getElementById('title');
    let line = document.getElementById('line');
    let count = 0;
    btn.onclick = function () {
        if(count%2===0){
            line.style.transform = "scaleX(1)";
            h2.style.animation = "changeColor1 1s";
            h2.style.color = "#188EA7";
            count++;
        }else{
            line.style.transform = "scaleX(0)";
            h2.style.animation = "changeColor2 1s";
            h2.style.color = "#000";
            count++;
        }
        
    }
})();