Text that shows an underline on hover

li {
 display:inline-block;
 padding:15px;
}

li:hover {
  color: Blue;
  border-bottom: 3px solid Blue;
}

<ul>
  <li>Hello work</li>
 </ul>

Fairly simple process I am using SCSS obviously but you don't have to as it's just CSS in the end!

HTML

<span class="menu">Menu</span>

SCSS

.menu {
    position: relative;
    text-decoration: none;
    font-weight: 400;
    color: blue;
    transition: all .35s ease;

    &::before {
        content: ""; 
        position: absolute;
        width: 100%;
        height: 2px;
        bottom: 0;
        left: 0;
        background-color: yellow;
        visibility: hidden;
        -webkit-transform: scaleX(0);
        transform: scaleX(0);
        -webkit-transition: all 0.3s ease-in-out 0s; 
        transition: all 0.3s ease-in-out 0s; 
    }   

    &:hover {
        color: yellow;

        &::before {
            visibility: visible;
            -webkit-transform: scaleX(1);
            transform: scaleX(1);
        }   
    }   
}

You just need to specify text-decoration: underline; with pseudo-class :hover.

HTML

<span class="underline-on-hover">Hello world</span>

CSS

.underline-on-hover:hover {
    text-decoration: underline;
}

I have whipped up a working Code Pen Demo.

<span class="txt">Some Text</span>

.txt:hover {
    text-decoration: underline;
}