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HTML

              
                %form
  %input{type: "text", required: ""}
  %label{placeholder: "Adaptive Placeholder", alt: "Placeholder"}
/ Featured on LittleBigDetails
/ http://littlebigdetails.com/post/82478225432/
              
            
!

CSS

              
                // First, pass in the important variables.
.adaptive_placeholder(@height, @radius, @margin: 1em, @border: 1px) {
  @borders: (@border * 2); // <- To avoid repeating below
  box-sizing: border-box;  // <- Important for accurate sizing
  width: 100%;             // <- Could be almost anything
  height: ~"calc(@{height} + @{borders})"; 
  margin: 0 0 @margin;
  padding: @margin;
  border: @border solid #ccc;
  border-radius: @radius;
  background: #fff;
  resize: none;  // <- For textareas
  outline: none; // <- Because ugly
  //
  // Following block controls all the color change
  //
  &[required] {
    &:focus {
      border-color: #00bafa;
      + label {
        &[placeholder] {
          &:before {
            color: #00bafa;
          }
        }
      }
    }
    //
    // Following block selects label directly adjacent to input
    //
    &:focus,
    &:valid {
      + label {
        &[placeholder] {
          &:before {
            transition-duration: .2s;
            //
            // Following lines move placeholder out of the way
            //
            transform: translate(0, (@margin * -1.5)) scale(.9, .9);
          }
        }
      }
    }
    //
    // Following block injects string from label's 'alt' attr.
    // It comes before the other 'content' property so I can
    // override it. Uses "invalid" pseudo-selector to know
    // when to do that.
    //
    &:invalid {
      + label {
        &[placeholder] {
          &[alt] {
            &:before {
              content: attr(alt); // Content property 1 of 2
            }
          }
        }
      }
    }
    + label {
      &[placeholder] {
        display: block;
        pointer-events: none; // Allows clicking thru label
        line-height: @margin * 1.25;
        // Following pulls label into position
        margin-top: ~"calc(-@{height} - @{borders})";
        // Following offsets that above
        margin-bottom: ~"calc((@{height} - @{margin}) + @{borders})";
        //
        // Following ':before' is needed to switch between
        // different strings.
        //
        &:before {
          content: attr(placeholder); // Content property 2 of 2
          display: inline-block;
          margin: 0 ~"calc(@{margin} + @{borders})";
          padding: 0 2px;
          color: #898989;
          white-space: nowrap;
          transition: .3s ease-in-out;
          //
          // Following lines lets me use solid color as bkg img.
          // This lets me size bkg to something more sublte
          //
          background-image: linear-gradient(to bottom, #fff, #fff);
          background-size: 100% 5px;
          background-repeat: no-repeat;
          background-position: center;
        }
      }
    }
  }
}
//
// Following block is how I call the above function
//
input {
  @height: 3em;
  &[type="text"] {
    .adaptive_placeholder(@height, (@height / 2));
  }
}










// For Demo Only
html {
  height: 100%;
}
body {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
}
              
            
!

JS

              
                
              
            
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999px

Console