Implementing Underline Animation Code

Hello,

Wondering how to go about implementing the "Underline 2" code: https://css-irl.info/animating-underlines/ to a single page. 

Thank you.

If you know CSS code, you can add a CSS pseudo with position: absolute, width: 0; height: 2px;

then when hover, use CSS to change width to 100%

If you don't know CSS, please share link to your site, we can give code easier