style.css

body {
  margin: 0 auto;
  box-sizing: border-box;
  width: 100%;
  max-width: 30em;
  padding: 1em;
  font-family: sans-serif;
}

body > * {
  margin-bottom: 1em;
}

.info {
  width: 100%;
  box-sizing: border-box;
  float: left;
  border: teal solid 1em;
  padding: 1em;
}

.info h1, p {
  margin: 0;
}

.shift {
  width: 100%;
  box-sizing: border-box;
  float: left;
  border: lightcoral solid 1em;
  padding: 1em;
}

.shift h3 {
  margin: 0;
  float: left;
}

.shift input {
  float: left;
  font-size: 1em;
  margin-left: 1em;
  margin-top: -0.2em;
}

.button {
  margin-left: .5em;
  background: lightcoral;
  height: 1em;
  width: 1em;
  float: left;
  font-size: 1.5em;
  text-align: center;
  margin-top: -0.1em;
  cursor: pointer;
}

.example {
  float: left;
  margin-left: .8em;
}

.input, .output {
  float: left;
  width: 100%;
  height: 10em;
  font-size: 1.2em;
  border: solid 1em;
  box-sizing: border-box;
  padding: 1em;
  resize: none;
}

.input {
  border-color: gold;
}

.output {
  border-color: thistle;
}

@media (min-width: 45em) {
  
  body {
    max-width: 80em;
  }
  
  .shift, .info {
    width: 30%;
    height: 16.8em;
    overflow: scroll;
  }
  
  .shift > * {
    float: none;
    clear: both;
    margin: 0;
  }
  
  .shift input {
    font-size: 1.6em;
    margin-top: .4em;
    margin-left: 0;
  }
  
  .shift .button {
    font-size: 2em;
    margin-top: .5em;
    float: left;
  }
  
  .shift .decrease {
    clear: none;
    margin-left: .5em;
  }
  
  .shift .example {
    font-size: 1.4em;
    padding-top: 1em;
  }
  
  .input, .output {
    width: calc(70% - 1em);
    height: 14em;
    margin-left: 1em;
  }
  
}