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HTML 

              
                #wrapper
  %form.form{action: ''}
    %p.field.required.half
      %label.label.required{for: 'name'} Name
      %input.text-input#name{type: 'text', name: 'name', required: true}
    %p.field.required.half
      %label.label{for: 'email'} E-mail
      %input.text-input#email{type: 'email', name: 'email', required: true}
    %p.field
      %label.label{for: "message"} Message
      %textarea.textarea#message{cols: 50, name: "message", rows: 4, required: true}
    %p.field
      %input.button{type: "submit", value: "Send message"}
!

CSS 

              
                // colors

$bg: white
$text: black

$label: rgba(black,.7)
$border: rgba(black,.3)

$active: black
$focus: black
$error: red
  
%block
  padding: 1em 1.5em
  appearance: none
  outline: none
  line-height: normal
  border: 1px solid transparent
  border-radius: 0

%input
  @extend %block
  font: inherit
  line-height: normal
  width: 100%
  box-sizing: border-box
  display: block

  padding-left: 0

  border-bottom-color: $border
  background: transparent
  outline: none

  color: $text

  &:placeholder
    color: $label

  &:-webkit-autofill
    box-shadow: 0 0 0px 1000px $bg inset
    border-top-color: $bg
    border-left-color: $bg
    border-right-color: $bg
  
  &.error, .error &
    border-color: transparent transparent $error transparent
  
  .form:not(.has-floated-label) &
    &:active, &:focus
      border-color: transparent transparent $text transparent

%floated
  @extend %block
  
  position: absolute
  z-index: 10
  pointer-events: none
  padding-left: 0
  
%floated-label
  @extend %floated
  
  top: 0
  left: 0
  color: $label
  transition: color .3s
  
  
  .active &
    font-size: .75em
    line-height: 1
    font-weight: 600
    text-transform: uppercase
    padding: 0
    color: $label
    background: $bg

  .focus &
    color: $text

  // .focus:not(.active) &
  //   transition: font .2s ease
  
.form.has-floated-label .field:after
  content: ''
  position: absolute
  bottom: 0
  left: 0
  width: 0%
  border-bottom: 1px solid $active
  transition: width .3s cubic-bezier(0.455, 0.030, 0.515, 0.955)

.form.has-floated-label .field.focus:after
  width: 100%


%button
  @extend %block
  
  font: inherit
  line-height: normal
  cursor: pointer

  background-color: $text
  color: white
  text-transform: uppercase
  text-align: center
  letter-spacing: #{(2/14)}em

  &:hover, &:focus, &:active
    color: white
    background: black
  

  &:active
    position: relative
    top: 1px
    left: 1px

.form

  max-width: 50em
  margin: 0 auto
  padding: 1em 2em
  box-sizing: border-box
  overflow: hidden
  
  .field
    position: relative
    width: 100%
    margin-bottom: 1.5em
    float: left
    &.half
      @media screen and (min-width: 40em)
        width: calc(50% - 2em)
        margin-right: 2em
        + .half
          margin:
            left: 2em
            right: 0
    &:last-child
      float: right
      width: auto

  .text-input
    @extend %input

  .textarea
    @extend %input
    max-width: 100%
    
  .button
    @extend %button
    
  .label
    @extend %floated-label

svg
  path
    fill: black
!

JS 

              
                class floatingLabel
  
  constructor: (form,options={}) ->
    
    return unless form
    
    options.focusClass ||= "focus"
    options.activeClass ||= "active"
    options.errorClass ||= "error"
    
    form.classList.add 'has-floated-label'
    
    for label in form.querySelectorAll('label')
      return unless input = document.querySelector("##{label.getAttribute('for')}")
                  
      for event in ['keyup','input', 'change']
        input.addEventListener event, ->
          @parentNode.classList.remove options.errorClass
          @parentNode.classList.toggle options.activeClass, !!@value
      input.addEventListener 'focus', ->
        @parentNode.classList.add options.focusClass
      input.addEventListener 'blur', ->
        @parentNode.classList.remove options.focusClass
      input.parentNode.classList.toggle options.activeClass, !!input.value

# initialize
    
window.floatingLabel = new floatingLabel document.querySelector('.form')
!
999px

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