<div class="content">
<div class="buttons">
        <button class="button" onclick="functionContentBox()">content-box</button>
  <button class="button" onclick="functionBorderBox()">border-box</button>  
  </div>
  
<div id="box-container">
  <div class="box1" id="box1">This is BOX1.</div>
  <div class="box2" id="box2">This is BOX2.</div>
  <div style="clear:both;"></div>
</div>
  
   <div class="col"><h2>CSS Code:</h2><p id="code">CSS code</p>
</div>

body {
  display: flex;
  height: 100vh;
  justify-content: center;
  align-items: center;
}

.content {
  top:-50px;
  position: relative;
  left:-70px;
}

#box-container {
  width: 300px;
  border: 3px solid #333;
  position: fixed;
}

#box1, #box2 {
  width: 150px;
  border: 3px solid #0a9396;
  float: left;
  padding: 10px;
}

.buttons {
  margin-left:-145px;
  top:60px;
  position: relative;
}

button {
  background-color: #333;
  color: white;
  border: none;
  display: block;
  padding:5px;
  width: 100px;
  margin:5px;
  transition: .2s;
  font-weight: bold;
}

button:hover {
  background-color: white;
  color: #333;
}

.col {
  position: relative;
  font-weight: bold;
  font-size:15px;
  width: 100%;
  left:345px;
  top:-20px;
}

function functionContentBox() {
  document.getElementById("box1").style.boxSizing = "content-box";
  document.getElementById("box2").style.boxSizing = "content-box";
  
  var x = document.getElementById("code");
  
  if (x.innerHTML === "box-sizing: content-box;") {
    x.innerHTML = "box-sizing: content-box;";
  } else {
    x.innerHTML = "box-sizing: content-box;";
  }
}
function functionBorderBox() {
  document.getElementById("box1").style.boxSizing = "border-box";
  document.getElementById("box2").style.boxSizing = "border-box";
  
  var x = document.getElementById("code");
  if (x.innerHTML === "box-sizing: border-box;") {
    x.innerHTML = "box-sizing: border-box;";
  } else {
    x.innerHTML = "box-sizing: border-box;";
  }
}