30 seconds of CSS

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Hover underline animation  animation

Creates an animated underline effect when the text is hovered over.

Credit: https://flatuicolors.com/

HTML
<p class="hover-underline-animation">Hover this text to see the effect!</p>
CSS
.hover-underline-animation {
  display: inline-block;
  position: relative;
  color: #0087ca;
}
.hover-underline-animation::after {
  content: '';
  position: absolute;
  width: 100%;
  transform: scaleX(0);
  height: 2px;
  bottom: 0;
  left: 0;
  background-color: #0087ca;
  transform-origin: bottom right;
  transition: transform 0.25s ease-out;
}
.hover-underline-animation:hover::after {
  transform: scaleX(1);
  transform-origin: bottom left;
}
Demo

Hover this text to see the effect!

Explanation
  1. display: inline-block makes the block p an inline-block to prevent the underline from spanning the entire parent width rather than just the content (text).
  2. position: relative on the element establishes a Cartesian positioning context for pseudo-elements.
  3. ::after defines a pseudo-element.
  4. position: absolute takes the pseudo element out of the flow of the document and positions it in relation to the parent.
  5. width: 100% ensures the pseudo-element spans the entire width of the text block.
  6. transform: scaleX(0) initially scales the pseudo element to 0 so it has no width and is not visible.
  7. bottom: 0 and left: 0 position it to the bottom left of the block.
  8. transition: transform 0.25s ease-out means changes to transform will be transitioned over 0.25 seconds with an ease-out timing function.
  9. transform-origin: bottom right means the transform anchor point is positioned at the bottom right of the block.
  10. :hover::after then uses scaleX(1) to transition the width to 100%, then changes the transform-origin to bottom left so that the anchor point is reversed, allowing it transition out in the other direction when hovered off.
Browser support

100.0%