JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang='en'>
  <head>
    <meta charset='UTF-8' />
    <title>Floats</title>
    <link rel='stylesheet' href='styles.css'/>
  </head>
  <body>
    <div class='page'>
      <div class='menu'>
           <div class="sidenav">
           <button class="dropdown-btn">Three Lines</button>
           <div>
           
           
           </div>
  <button class="dropdown-btn">Region</button>
  <div class="dropdown-container">
    <ul id="regionList">
      <li>US</li>
      <li>Japan</li>
      <li>Europe</li>
    </ul>
  </div>
  <button class="dropdown-btn">Export</button>
  <div class="dropdown-container">
    <a href="#">Link 1</a>
    <a href="#">Link 2</a>
    <a href="#">Link 3</a>
  </div>
</div>
      </div>
      <div class='sidebar'>Sidebar</div>
      <div class='content'>Content</div>
      <div class='footer'>Footer</div>
    </div>
  </body>
</html>

CSS

.menu {
  height: 100px;
  background-color: #B2D6FF;    /* Medium blue */
}

.sidebar {
  height: 300px;
  background-color: #F09A9D;    /* Red */
}

.content {
  height: 500px;
  background-color: #F5CF8E;    /* Yellow */
}

.footer {
  height: 200px;
  background-color: #D6E9FE;    /* Light blue */
}

.sidenav {
  width: 20%;
  position: relative;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: #111;
  overflow-x: hidden;
  padding-top: 20px;
}

.sidenav li, .dropdown-btn {
  padding: 6px 8px 6px 16px;
  text-decoration: none;
  font-size: 20px;
  color: #818181;
  display: block;
  border: none;
  background: none;
  width:100%;
  text-align: left;
  cursor: pointer;
  outline: none;
}

.sidenav a, .dropdown-btn {
  padding: 6px 8px 6px 16px;
  text-decoration: none;
  font-size: 20px;
  color: #818181;
  display: block;
  border: none;
  background: none;
  width:100%;
  text-align: left;
  cursor: pointer;
  outline: none;
}

/* On mouse-over */
.sidenav a:hover, .dropdown-btn:hover {
  color: #f1f1f1;
}

/* Main content */
.main {
  margin-left: 200px; /* Same as the width of the sidenav */
  font-size: 20px; /* Increased text to enable scrolling */
  padding: 0px 10px;
}

/* Add an active class to the active dropdown button */
.active {
  background-color: green;
  color: white;
}

/* Dropdown container (hidden by default). Optional: add a lighter background color and some left padding to change the design of the dropdown content */
.dropdown-container {
  display: none;
  background-color: #262626;
  padding-left: 8px;
}

/* Optional: Style the caret down icon */
.fa-caret-down {
  float: right;
  padding-right: 8px;
}

JavaScript

var dropdown = document.getElementsByClassName("dropdown-btn");
var i;

for (i = 0; i < dropdown.length; i++) {
  dropdown[i].addEventListener("click", function() {
    this.classList.toggle("active");
    var dropdownContent = this.nextElementSibling;
    if (dropdownContent.style.display === "block") {
      dropdownContent.style.display = "none";
    } else {
      dropdownContent.style.display = "block";
    }
  });
}