解决方案：首先尝试将边框设置为透明,因此它在那里但无法看到：

a{ 
    border-bottom: 2px solid transparent; /* <- here */
    transition: border-bottom 1s;
    text-decoration: none; /* I added this for clarity of effect */
}
a:hover{
    border-bottom: 2px solid red;
}

<a href="">testing border</a>