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HTML 

              
                <section class="wrapper">
	<div class="box">
		<h1>From left</h1>
		<div class="st-form__input--wrapper from-left">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From right</h1>
		<div class="st-form__input--wrapper from-right">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From top</h1>
		<div class="st-form__input--wrapper from-top">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From bottom</h1>
		<div class="st-form__input--wrapper from-bottom">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From sides</h1>
		<div class="st-form__input--wrapper from-side">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From center</h1>
		<div class="st-form__input--wrapper from-center">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>From middle</h1>
		<div class="st-form__input--wrapper from-middle">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Blink</h1>
		<div class="st-form__input--wrapper blink">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Half</h1>
		<div class="st-form__input--wrapper half">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Circle</h1>
		<div class="st-form__input--wrapper circle">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
			<span class="input-border"></span>
		</div>
  </div>

	<div class="box">
		<h1>Bottom border</h1>
		<div class="st-form__input--wrapper just-bottom">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Top / Bottom border</h1>
		<div class="st-form__input--wrapper top-bottom">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Partially Top / Bottom border</h1>
		<div class="st-form__input--wrapper partially">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
			<span class="input-border"></span>
		</div>
	</div>

	<div class="box">
		<h1>Enlarge</h1>
		<div class="st-form__input--wrapper enlarge">
			<input type="text" class="st-form__input text" />
			<span class="input-border"></span>
		</div>
	</div>
</section>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro:200,400,600);

$black: rgb(0, 0, 0);
$white: lighten($black, 100%);

$dark-blue: rgb(46, 64, 72);
$blue: rgb(1, 175, 209);
$orange: rgb(250, 179, 43);
$medium-green: rgb(55, 200, 123);

$alternate-gray: rgb(250, 250, 250);
$light-gray: rgb(242, 242, 242);
$gray: rgb(212, 212, 212);
$dark-gray: rgb(136, 136, 136);
$turqouise: rgb(18, 121, 142);
$medium-blue: rgb(0, 75, 104);

$yellow: rgb(250, 217, 33);
$red: rgb(208, 61, 60);
$green: rgb(114, 152, 15);
$light-orange: rgb(255, 246, 229);

$anim-speed: 0.3s;
$input-height: 40px;
$input-space: 20px;
$border-width: 2px;
$border-color: $blue;

// Screen size
// --------------------------------------------------------------------------------
$screen-xs: 480px;
$screen-xs-min: $screen-xs;

$screen-sm: 768px;
$screen-sm-min: $screen-sm;

$screen-md: 992px;
$screen-md-min: $screen-md;

$screen-lg: 1200px;
$screen-lg-min: $screen-lg;

$screen-xs-max: ($screen-sm-min - 1);
$screen-sm-max: ($screen-md-min - 1);
$screen-md-max: ($screen-lg-min - 1);


// Cols
// --------------------------------------------------------------------------------
$num-of-cols: 12;
$col-1: percentage(1 / $num-of-cols);
$col-2: percentage(2 / $num-of-cols);
$col-3: percentage(3 / $num-of-cols);
$col-4: percentage(4 / $num-of-cols);
$col-5: percentage(5 / $num-of-cols);
$col-6: percentage(6 / $num-of-cols);
$col-7: percentage(7 / $num-of-cols);
$col-8: percentage(8 / $num-of-cols);
$col-9: percentage(9 / $num-of-cols);
$col-10: percentage(10 / $num-of-cols);
$col-11: percentage(11 / $num-of-cols);
$col-12: percentage(12 / $num-of-cols);

// Mixins
// --------------------------------------------------------------------------------
@mixin size($height, $width) {
  height: $height;
  width: $width;
}

@mixin square($size) {
  @include size($size, $size);
}

@keyframes blink {
  0% {opacity: 0;}
  50% {opacity: 1;}
  55% {opacity: 0;}
  100% {opacity: 1;}
}

* {
  background-repeat: no-repeat;
  box-sizing: border-box;
  font-family: 'Source Sans Pro', sans-serif;
  outline: 0;

  &:before, &:after {
    background-repeat: no-repeat;
    box-sizing: border-box;  
    outline: 0;
  }
}

html, body {
  height: 100%;
}

body {
  background-color: $light-gray;
  font-family: 'Source Sans Pro', sans-serif;
  color: $dark-blue;
  margin: 10px;
}

.wrapper {
  display: flex;
  flex-wrap: wrap;
}

.box {
  flex: 0 0 $col-6;
  padding: 0 10px;
}

h1 {
  font-weight: 400;
  margin-bottom: 10px;
}

.st-form__input--wrapper {
  height: $input-height;
  margin-bottom: $input-space;
  position: relative;
  width: $col-12;

  input[type="text"] {
    @include square($col-12);
    background-color: $white;
    border: solid 1px $gray;
    font-weight: 400;
    padding: 0 10px;
    position: relative;
    z-index: 10;

    &:focus {
      background-color: transparent;
      border-color: transparent;

      & ~ .input-border {
        background-color: $white;

        &:before,
        &:after {
        }
      }
    }
  }

  .input-border {
    bottom: 0;
    display: block;
    left: 0;
    position: absolute;
    top: 0;
    width: 0;

    &:before, &:after {
      bottom: 0;
      content: '';
      display: block;
      position: absolute;
      top: 0;
      width: 0;
      transition: $anim-speed ease-in-out all;
    }

    &:after {
      display: none;
    }
  }

  &.from-left {

    input[type="text"] {

      &:focus ~ .input-border {
        width: 100%;

        &:before,
        &:after {
          width: 100%;
        }
      }
    }

    .input-border {

      &:before {
        box-shadow: inset 0 0 0 $border-width $border-color;
        left: 0;
      }
    }
  }

  &.from-right {

    input[type="text"] {

      &:focus ~ .input-border {
        width: 100%;

        &:before,
        &:after {
          width: 100%;
        }
      }
    }

    .input-border {

      &:before {
        box-shadow: inset 0 0 0 $border-width $border-color;
        right: 0;
      }
    }
  }

  &.from-top {

    input[type="text"] {

      &:focus ~ .input-border {
        width: 100%;

        &:before,
        &:after {
          height: 100%;
        }
      }
    }

    .input-border {

      &:before {
        box-shadow: inset 0 0 0 $border-width $border-color;
        height: 0;
        width: 100%;
      }
    }
  }

  &.from-bottom {

    input[type="text"] {

      &:focus ~ .input-border {
        width: 100%;

        &:before,
        &:after {
          height: 100%;
        }
      }
    }

    .input-border {

      &:before {
        box-shadow: inset 0 0 0 $border-width $border-color;
        top: auto;
        height: 0;
        width: 100%;
      }
    }
  }

  &.from-side {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        width: 100%;

        &:before,
        &:after {
          width: 50%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 0;
        transition: $anim-speed ease-in-out all;
      }

      &:before {
        border: solid $border-width $border-color;
        border-right: 0;
        left: 0;
      }

      &:after {
        border: solid $border-width $border-color;
        border-left: 0;
        right: 0;
      }
    }
  }

  &.from-center {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        width: 100%;

        &:before,
        &:after {
          width: 50%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 0;
        transition: $anim-speed ease-in-out all;
      }

      &:before {
        border: solid $border-width $border-color;
        border-left: 0;
        left: 50%;
      }

      &:after {
        border: solid $border-width $border-color;
        border-right: 0;
        right: 50%;
      }
    }
  }

  &.from-middle {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        height: 100%;
        width: 100%;

        &:before,
        &:after {
          height: 100%;
          opacity: 1;
          width: 50%;
        }
      }
    }

    .input-border {
      height: 0;
      opacity: 0;
      top: 50%;
      transform: translateY(-50%);
      width: 100%;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        height: 0;
        position: absolute;
        opacity: 0;
        top: 50%;
        transform: translateY(-50%);
        width: 0;
        transition: $anim-speed ease-in-out all;
      }

      &:before {
        border: solid $border-width $border-color;
        border-left: 0;
        left: 50%;
      }

      &:after {
        border: solid $border-width $border-color;
        border-right: 0;
        right: 50%;
      }
    }
  }

  &.blink {

    input[type="text"] {

      &:focus ~ .input-border {
        animation: blink $anim-speed forwards 1 alternate;
        width: 100%;

        &:before,
        &:after {
          width: 100%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 100%;

      }

      &:before {
        border: solid $border-width $border-color;
        width: 100%;
      }

      &:after {
        display: none;
      }
    }
  }

  &.half {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        width: 100%;

        &:before,
        &:after {
          width: 100%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 0;
        transition: ($anim-speed * 2) ease-in-out all;
      }

      &:before {
        border-left: solid $border-width $border-color;
        border-top: solid $border-width $border-color;
        left: 0%;
      }

      &:after {
        border-right: solid $border-width $border-color;
        border-bottom: solid $border-width $border-color;
        right: 0%;
      }
    }
  }

  &.circle {

    input[type="text"] {

      &:focus ~ .input-border {

        &:first-of-type {

          &:before {
            transition: ($anim-speed / 2) ease-in-out all;
            transition-delay: 0s;
            width: $col-12;
          }

          &:after {
            height: $col-12;
            transition: ($anim-speed / 2) ease-in-out all;
            transition-delay: ($anim-speed / 2);
          }
        }

        &:last-of-type {
          background: transparent;

          &:before {
            transition: ($anim-speed / 2) ease-in-out all;
            transition-delay: $anim-speed;
            width: $col-12;
          }

          &:after {
            height: $col-12;
            transition: ($anim-speed / 2) ease-in-out all;
            transition-delay: (($anim-speed / 2) + $anim-speed);
            top: 0
          }
        }
      }
    }

    .input-border {
      bottom: auto;
      height: $col-12;
      width: $col-12;

      &:before,
      &:after {
        background-color: $border-color;
        content: '';
        display: block;
        position: absolute;
      }

      &:first-of-type {

        &:before {
          height: $border-width;
          left: 0;
          top: 0;
          width: 0;
        }

        &:after {
          height: 0;
          right: 0;
          top: 0;
          width: $border-width;
        }
      }

      &:last-of-type {

        &:before {
          background-color: $border-color;
          height: $border-width;
          top: calc(#{$col-12} - #{$border-width});
          right: 0;
          width: 0;
        }

        &:after {
          background-color: $border-color;
          height: 0;
          top: calc(#{$col-12} - #{$border-width});
          left: 0;
          width: $border-width;
        }
      }
    }
  }

  &.just-bottom {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        width: 100%;

        &:before,
        &:after {
          width: 50%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 0;
        transition: $anim-speed ease-in-out all;
      }

      &:before {
        border-bottom: solid $border-width $border-color;
        left: 50%;
      }

      &:after {
        border-bottom: solid $border-width $border-color;
        right: 50%;
      }
    }
  }

  &.top-bottom {

    input[type="text"] {

      &:focus ~ .input-border {
        opacity: 1;
        width: 100%;

        &:before,
        &:after {
          width: 50%;
        }
      }
    }

    .input-border {
      opacity: 0;

      &:before, &:after {
        box-shadow: none;
        bottom: 0;
        content: '';
        display: block;
        position: absolute;
        top: 0;
        width: 0;
        transition: $anim-speed ease-in-out all;
      }

      &:before {
        border-bottom: solid $border-width $border-color;
        border-top: solid $border-width $border-color;
        border-right: solid 1px $white;
        left: 50%;
      }

      &:after {
        border-bottom: solid $border-width $border-color;
        border-top: solid $border-width $border-color;
        border-left: solid 1px $white;
        right: 50%;
      }
    }
  }

  &.partially {

    input[type="text"] {

      &:focus ~ .input-border {

        &:first-of-type {
          border-left: solid $border-width $border-color;
        }

        &:last-of-type {
          background: transparent;
          border-right: solid $border-width $border-color;
        }

        &:before,
        &:after {
          height: $border-width;
          width: ($col-1 / 2);
        }
      }
    }

    .input-border {
      bottom: auto;
      height: $col-12;
      width: $col-12;

      &:before,
      &:after {
        background-color: $border-color;
        content: '';
        display: block;
        position: absolute;
      }

      &:before {
        top: 0;
      }

      &:after {
        bottom: 0;
        top: auto;
      }

      &:first-of-type {
        left: 0;

        &:before,
        &:after {
          left: 0;
        }
      }

      &:last-of-type {
        right: 0;

        &:before,
        &:after {
          right: 0;
        }
      }
    }
  }

  &.enlarge {

    input[type="text"] {

      &:focus {
        background-color: $white;
        border-color: $border-color;
        border-width: 2px;
        transition: ($anim-speed * 0.5) ease-in-out all;
      }
    }

    .input-border {
      display: none;
    }
  }
}
!

JS
!
999px

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