CodePen

HTML

            
              <div class='screen'>
  
  <img class='logo' src='http://s.cdpn.io/15065/twitter-bird-dark-bgs.png' />
  
  <form action='' class='login cf'>
    
    <p class='large'>Welcome</p>
    
    <input type='text' placeholder='Username or Email' autofocus />
    
    <input type='password' placeholder='Password' />
    
    <a class='small flo-l' href=''>Did you forget?</a>
    
    <div class='flo-r cf'>
      <input class='flo-l' type='checkbox' id='remember-me' />
      <label class='small flo-l' for='remember-me'>Remember me</label>
    </div>
    <div class='signup-link flo-l clr-l'>
      <p class='small'>New to Twitter?</p>
      <a class='medium' href=''>Sign Up</a>
    </div>
    
    <input class='flo-r' type='submit' value='Sign In' />
    
  </form>
  
</div>
            
          
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CSS

            
              $black: #252525;
$blue: #00A0D1;

* {
  margin: 0;
  padding: 0;
  -moz-box-sizing: border-box;
       box-sizing: border-box;
}

      a { text-decoration:none }
a:hover { text-decoration:underline }

.flo-l { float:left }
.flo-r { float:right }
.clr-b { clear:both }
.clr-l { clear:left }
   .cf { overflow:hidden }

input[type='submit'],
input[type='checkbox'],
input[type='checkbox'] + label { cursor:pointer }

body, html { height:100%; width:100% }

body {
  background: lightgray;
  font-family: sans-serif;
}

.screen {
  position: relative;
  background: $black;
  margin: 0 auto;
  width: 100%;
  max-width: 320px;
  min-width: 295px;
  height: 100%;
  max-height: 515px;
  min-height: 480px;
}

.logo {
  display: block;
  position: relative;
  width: 70%;
  margin: 0 auto;
}

.small {
  font-size: 0.72em;
  color: hsl(0, 0%, 40%);
}

label.small:hover { color:hsl(0, 0%, 50%) }

.large {
  color: #fff;
  font-size: 1.3em;
  line-height: 1em;
  font-family: Georgia, serif;
  font-weight: 700;
  margin-bottom: 0.6em;
}

.medium {
  font-size: 0.8em;
  color: hsl(0, 0%, 60%);
  
  &:hover { color:hsl(0, 0%, 70%) }
}

.signup-link {
  margin-top: 1.4em;
}

.login {
  position: absolute;
  bottom: 0;
  width: 100%;
  padding: 0 1.5em 1.5em;

  input[type='text'],
  input[type='password'] {
    width: 100%;
    margin-bottom: 1.5em;
    border: 0;
    border-radius: 3px;
    height: 2.2em;
    font-size: 1.2em;
    color: hsl(0, 0%, 40%);
    padding-left: 0.5em;
  }
  
  input[type='password'] {
    margin-bottom: 0.7em;
  }
  
  input[type='submit'] {
    margin-top: 0.7em;
    background: $blue;
    background: linear-gradient(lighten($blue, 2.5%), darken($blue, 2.5%));
    border-radius: 3px;
    border: 0;
    height: 2em;
    color: #fff;
    font-family: Georgia, serif;
    font-size: 1.3em;
    font-weight: 700;
    padding: 0 0.5em;
    
    &:hover {
      background: linear-gradient(lighten($blue, 7.5%), lighten($blue, 2.5%));
    }
  }
  
  label.small {
    padding-left: 0.5em;
  }
}
            
          
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