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CSS

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JavaScript

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HTML 

              
                div.login
  header.login-header
    span.text LOGIN
    span.loader
  form.login-form
    input.login-input(type="text", placeholder="Username")
    input.login-input(type="password", placeholder="Password")
    button.login-btn(type="submit") login
!

CSS 

              
                $size: 20rem

*
  -moz-box-sizing: border-box
  box-sizing: border-box

body
  background: #333
  font: 100%/1 "Helvetica Neue", Arial, sans-serif
  
.login
  position: absolute
  top: 50%
  left: 50%
  margin: #{-$size / 2} 0 0 #{-$size / 2}
  width: $size
  height: $size
  padding: 20px
  background: #fff
  border-radius: 5px
  overflow: hidden
  &:hover, &.focused
    > .login-header
      width: 2rem
      > .text
        font-size: 1rem
        transform: rotate(-90deg)
  &.loading
    > .login-header
      width: $size
      > .text
        display: none
      > .loader
        display: block
    
.login-header
  position: absolute
  left: 0
  top: 0
  z-index: 1
  width: $size
  height: $size
  background: orange
  transition: width 0.5s ease-in-out
  > .text
    display: block
    width: 100%
    height: 100%
    font-size: 5rem
    text-align: center
    line-height: $size
    color: #fff
    transition: all 0.5s ease-in-out
  > .loader
    display: none
    position: absolute
    left: 5rem
    top: 5rem
    width: 10rem
    height: 10rem
    border: 2px solid #fff
    border-radius: 50%
    animation: loading 2s linear infinite
    &:after
      content: ""
      position: absolute
      left: 4.5rem
      top: -0.5rem
      width: 1rem
      height: 1rem
      background: #fff
      border-radius: 50%
      border-right: 2px solid orange
    &:before
      content: ""
      position: absolute
      left: 4rem
      top: -0.5rem
      width: 0
      height: 0
      border-right: 1rem solid #fff
      border-top: 0.5rem solid transparent
      border-bottom: 0.5rem solid transparent

@keyframes loading
  50%
    opacity: 0.5
  100%
    transform: rotate(360deg)

.login-form
  margin: 0 0 0 2rem
  padding: 0.5rem

.login-input
  display: block
  width: 100%
  font-size: 2rem
  padding: 0.5rem 1rem
  box-shadow: none
  border-color: #ccc
  border-width: 0 0 2px 0
  & + &
    margin: 10px 0 0
  &:focus
    outline: none
    border-bottom-color: orange

.login-btn
  $btn-size: 5rem
  $btn-border: 0.6rem
  $triangle-width: 2.4rem
  $triangle-height: 1.2rem
  position: absolute
  right: 1rem
  bottom: 1rem
  width: $btn-size
  height: $btn-size
  border: none
  background: orange
  border-radius: 50%
  font-size: 0
  border:  $btn-border solid transparent
  transition: all 0.3s ease-in-out
  &:after
    content: ""
    position: absolute
    left: 1rem
    top: 0.8rem
    width: 0
    height: 0
    border-left: $triangle-width solid #fff
    border-top: $triangle-height solid transparent
    border-bottom: $triangle-height solid transparent
    transition: border 0.3s ease-in-out 0s
  &:hover, &:focus, &:active
    background: #fff
    border-color: orange
    outline: none
    &:after
      border-left-color: orange
!

JS 

              
                $('.login-input').on('focus', function() {
  $('.login').addClass('focused');
});

$('.login').on('submit', function(e) {
  e.preventDefault();
  $('.login').removeClass('focused').addClass('loading');
});
!
999px

Console