Sign In/Up Transition By Fabrizio Bianchi : http://codepen.io/fbrz/pen/pvZRNK

css

@import url(http://fonts.googleapis.com/css?family=Raleway:700,800);

html, body { margin: 0; }

:focus { outline: none; }
::-webkit-input-placeholder { color: #DEDFDF; }
::-moz-placeholder { color: #DEDFDF; }
:-moz-placeholder { color: #DEDFDF; }
::-ms-input-placeholder { color: #DEDFDF; }

body {
  background: #6ED0F6;
  color: #fff;
  font-family: 'Raleway', sans-serif;
  -webkit-font-smoothing: antialiased;
}

#wrapper, label, #arrow, button span { -webkit-transition: all .5s cubic-bezier(.6,0,.4,1); transition: all .5s cubic-bezier(.6,0,.4,1); }

#wrapper { overflow: hidden; }

#signin:checked ~ #wrapper { height: 178px; }
#signin:checked ~ #wrapper #arrow { left: 32px; }
#signin:checked ~ button span { -webkit-transform: translate3d(0,-72px,0); transform: translate3d(0,-72px,0); }

#signup:checked ~ #wrapper { height: 262px; }
#signup:checked ~ #wrapper #arrow { left: 137px; }
#signup:checked ~ button span { -webkit-transform: translate3d(0,-144px,0); transform: translate3d(0,-144px,0); }

#reset:checked ~ #wrapper { height: 94px; }
#reset:checked ~ #wrapper #arrow { left: 404px; }
#reset:checked ~ button span { -webkit-transform: translate3d(0,0,0); transform: translate3d(0,0,0); }

form {
  width: 450px;
  height: 370px;
  margin: -185px -225px;
  position: absolute;
  left: 50%;
  top: 50%;
}

input[type=radio] { display: none; }

label {
  cursor: pointer;
  display: inline-block;
  font-size: 22px;
  font-weight: 800;
  opacity: .5;
  margin-bottom: 30px;
  text-transform: uppercase;
}
label:hover {
  -webkit-transition: all .3s cubic-bezier(.6,0,.4,1);
          transition: all .3s cubic-bezier(.6,0,.4,1);
  opacity: 1;
}
label[for="signin"] { margin-right: 20px; }
label[for="reset"] { float: right; }
input[type=radio]:checked + label { opacity: 1; }

input[type=text],
input[type=password] {
  background: #fff;
  border: none;
  border-radius: 8px;
  font-size: 27px;
  font-family: 'Raleway', sans-serif;
  height: 72px;
  width: 99.5%;
  margin-bottom: 10px;
  opacity: 1;
  text-indent: 20px;
  -webkit-transition: all .2s ease-in-out;
          transition: all .2s ease-in-out;
}
button {
  background: #079BCF;
  border: none;
  border-radius: 8px;
  color: #fff;
  cursor: pointer;
  font-family: 'Raleway', sans-serif;
  font-size: 27px;
  height: 72px;
  width: 100%;
  margin-bottom: 10px;
  overflow: hidden;
  -webkit-transition: all .3s cubic-bezier(.6,0,.4,1);
          transition: all .3s cubic-bezier(.6,0,.4,1);
}
button span {
  display: block;
  line-height: 72px;
  position: relative;
  top: -2px;
  -webkit-transform: translate3d(0,0,0);
          transform: translate3d(0,0,0);
}
button:hover {
  background: #007BA5;
}

#arrow {
  height: 0;
  width: 0;
  border-bottom: 10px solid #fff;
  border-left: 10px solid transparent;
  border-right: 10px solid transparent;
  position: relative;
  left: 32px;
}


#hint {
  width: 100%;
  text-align: center;
  position: absolute;
  bottom: 20px;
}

html

<form>
  <input checked id='signin' name='action' type='radio' value='signin'>
  <label for='signin'>Sign in</label>
  <input id='signup' name='action' type='radio' value='signup'>
  <label for='signup'>Sign up</label>
  <input id='reset' name='action' type='radio' value='reset'>
  <label for='reset'>Reset</label>
  <div id='wrapper'>
    <div id='arrow'></div>
    <input id='email' placeholder='Email' type='text'>
    <input id='pass' placeholder='Password' type='password'>
    <input id='repass' placeholder='Repeat password' type='password'>
  </div>
  <button type='submit'>
    <span>
      Reset password
      <br>
      Sign in
      <br>
      Sign up
    </span>
  </button>
</form>
<div id='hint'>Click on the tabs</div>