<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. <a>Mollitia nostrum placeat consequatur deserunt velit ducimus possimus commodi temporibus debitis quam</a>, molestiae laboriosam sit repellendus sed sapiente quidem quod accusantium vero.</p>

html,
body {
    width: 100%;
    height: 100%;
    display: flex;
}

p {
    width: 600px;
    margin: auto;
    font-size: 24px;
    line-height: 2;
    color: #666;
}

a {
    background: linear-gradient(90deg, #ff3c41, #fc0, #0ebeff);
    background-size: 0 3px;
    background-repeat: no-repeat;
    background-position: 0 100%;
    // background-position: 100% 100%;
    cursor: pointer;
    transition: 1s all;
    color: #0cc;
}

a:hover,
a:active {
    background-size: 100% 3px;
    color: #000;
}