Check out example codes for "outline bottom css". It will help you in understanding the concepts better.

Code Example 1

div {
    width: 100px;
    height: 100px;
    background: #eee;
}
div:hover {
    width: 100px;
    height: 100px;
    background: #eee;
    border-bottom: 1px solid;
    margin-bottom: -1px;
}

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