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  <input placeholder="Username" type="text">
  <input placeholder="Password" type="password">
	<textarea placeholder="Description"></textarea>



                form {
	text-align: center;
	width: 50%;
	margin: 0 auto;

textarea {
	display: block;
	width: 100%;
	border: 0;
	padding: 10px 5px;
  background: white no-repeat;
  /* 2 imgs : 1px gray line (normal state) AND 2px green line (focus state) */
	background-image: linear-gradient(to bottom, #1abc9c, #1abc9c), linear-gradient(to bottom, silver, silver);
	/* sizes for the 2 images (default state) */
	background-size: 0 2px, 100% 1px;
	/* positions for the 2 images. Change both "50%" to "0%" or "100%" and tri again */
	background-position: 50% 100%, 50% 100%;

	/* animation solely on background-size */
  transition: background-size 0.3s cubic-bezier(0.64, 0.09, 0.08, 1);


  /* sizes for the 2 images (focus state) */
	background-size: 100% 2px, 100% 1px;
	outline: none;


We use two background gradients, for two states. Each background visibility is ruled by the background-size.

- default state: 1px silver bottom background is visible.
- focus state: 2px color bottom background is visible

The size of the background is animated with "transition". The y-size remains always the same. The x-size is changed.

- default state
  - silver background is 100% wide
  - color background is 0% wide.
- focus state
  - silver background is 0% wide
	- color background is 100% wide

Animation is solely on the background-size, making it grow from 0 to 100% on focus and shrink back to 0% on blur.

The grow/shrink goes from the center to the borders.
To make it go from/to a side, replace the "50% 100%" with "0 100%" or "100% 100%" (for left or right respectively) on the background-position.

This works on <input> and on <textarea> alike.