kitchn/recipes/src/styles/_ui.scss

$box-shadow: 0 2px 2px 0 rgba(0,0,0,.16), 0 0 2px 0 rgba(0,0,0,.12);
$phone: "(max-width: 839px)";

@mixin button-reset {
  border: none;
  font: inherit;
  color: inherit;
  background-color: transparent;
  cursor: pointer;
  box-shadow: 0;
  outline: 0;
}

@mixin lightInput() {
  width: 100%;
  position: relative;
  margin-bottom: $spacing-regular;

  input,
  textarea {
    width: 100%;
    border: 0;
    padding: 4px 0;
    border-bottom: 1px solid #ccc;
    background-color: transparent;
  }

  span {
    position: absolute;
    display: block;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    pointer-events: none;
  }
  
  span::before {
    content: "";
    display: block;
    position: absolute;
    bottom: 0px;
    left: 0;
    width: 0;
    height: 1px;
  
    -webkit-transition: all 0.4s;
    -o-transition: all 0.4s;
    -moz-transition: all 0.4s;
    transition: all 0.4s;
  
    background: #7f7f7f;
  }
  
  input:focus + span::before,
  textarea:focus + span::before {
    width: 100%;
  }
  
  input.has-value + span::before,
  textarea.has-value + span::before {
    width: 100%;
  }
}