Edit in JSFiddle

<!DOCTYPE html>

  <html>

    <head>

      <meta charset="utf-8">
      <meta name="viewport" content="width=device-width, initial-scale=1">
      <link rel="stylesheet" href="styles.css">
      <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.6.3/css/font-awesome.min.css">
      
      <title>Diana Badas</title>

    </head>

    <body>
    
      <!-- Navigation -->
      <ul>
        <li><a href="#">Portfolio</a></li>
        <li><a href="#">About</a></li>
        <li><a href="#">Contact</a></li>
        <li style="float:left"><a class="active" href="#">Diana Badas</a></li>
      </ul>

       <!-- Slide Show -->
      <section>
        <img class="mySlides" src = "https://s5.postimg.cc/3prpgvdlj/pexels-photo-774731.jpg" style="width:100%">
        <img class="mySlides" src="https://s5.postimg.cc/dzu4g4b6v/pexels-photo-937596.jpg" style="width:100%">
      </section>

      <!-- Main Text -->
      <section  class="mainTextContainer">
        <h1 class="intro">My projects</h1>
        <p class="important"><i>Find below a selection of my projects </i></p>
        <p class="generalText">More info</p>
      </section>

      <!-- Articles -->
      <section class="container">
        <article class="leftpane">
        <h1 class="intro">Project x</h1>
        </article>
        <article class="middlepane">
        <h1 class="intro">Project Y</h1>
        </article>
        <article class="rightpane">
        <h1 class="intro">Project Z</h1>
        </article>
      </section>    
     
      <!-- Footer -->
      <footer class="footer">
        <h1 class="intro">This is my footer</h1>
        <a href="#"><i class="fa fa-facebook-official"></i></a>
        <a href="#"><i class="fa fa-pinterest-p"></i></a>
        <a href="#"><i class="fa fa-twitter"></i></a>
        <a href="#"><i class="fa fa-flickr"></i></a>
        <a href="#"><i class="fa fa-linkedin"></i></a>
      </footer>

      <script>
        // Automatic Slideshow - change image every 5 seconds
        var myIndex = 0;
        carousel();
            
        function carousel() {
          var i;
          var x = document.getElementsByClassName("mySlides");
          for (i = 0; i < x.length; i++) {
            x[i].style.display = "none";
          }
          myIndex++;
          if (myIndex > x.length) {myIndex = 1}
          x[myIndex-1].style.display = "block";
          setTimeout(carousel, 5000);
        }
      </script>
    
    </body>
  </html>
ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  border: 1px solid #e7e7e7;
  background-color: #f3f3f3;
  font-family:"Courier New";
}
            
li {
  float: right;
}
            
li a {
  display: block;
  color: rgb(134, 131, 133);
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
  font-family:"Courier New";
}
            
li a:hover:not(.active) {
  background-color: rgb(238, 206, 228);
}
            
li a.active {
  color: white;
  background-color: rgb(228, 125, 211);
 }

h1.intro {
  color: black;
  text-align:center;
  font-family:"Courier New";
}

p.important {
  color: gray;
  text-align:center;
  font-family:"Courier New";
}

p.generalText {
  color: black;
  text-align:center;
  font-family:"Courier New";
}

.mainTextContainer {
  width:100%;
  height:100%;
  background-color: rgb(159, 159, 216);
}

.textContainerPortfolio {
  width:100%;
  height:25%;
}

h2, p {
  font-family:"Courier New";
}

body, html {
  width: 100%;
  height: 100%;
  margin: 0;
}

* {
  box-sizing: border-box;
}

/* Style container Home page */
.container {
  width: 100%;
  height: 100%;
}

.leftpane {
  width: 25%;
  height: 100%;
  float: left;
  background-color: aliceblue;
  border-collapse: collapse;
}

.middlepane {
  width: 50%;
  height: 100%;
  float: left;
  background-color: antiquewhite;
  border-collapse: collapse;
}

.rightpane {
  width: 25%;
  height: 100%;
  position: relative;
  float: right;
  background-color: azure;
  border-collapse: collapse;
}

.footer {
  left: 0;
  bottom: 0;
  width: 100%;
  background-color: #f3f3f3;
  color: rgb(134, 131, 133);
  text-align: center;
}

/* Style inputs */
input[type=text], select, textarea {
  width: 100%;
  padding: 12px;
  border: 1px solid #ccc;
  margin-top: 6px;
  margin-bottom: 16px;
  resize: vertical;
  font-family:"Courier New";
}

input[type=submit] {
  background-color: rgb(228, 125, 211);
  color: white;
  padding: 12px 20px;
  border: none;
  cursor: pointer;
  font-family:"Courier New";
}

input[type=submit]:hover {
  background-color: blueviolet;
  font-family:"Courier New";
}

/* Style the container/contact section */
.containerContactPage {
  border-radius: 5px;
  background-color: #f2f2f2;
  padding: 10px;
}

/* Create two columns that float next to eachother */
.column {
  float: left;
  width: 50%;
  margin-top: 6px;
  padding: 20px;
  font-family:"Courier New";
}

/* Clear floats after the columns */
.row:after {
  content: "";
  display: table;
  clear: both;
}

/* Responsive layout - when the screen is less than 600px wide, make the two columns stack on top of each other instead of next to each other */
@media screen and (max-width: 600px) {
  .column, input[type=submit] {
      width: 100%;
      margin-top: 0;
      font-family:"Courier New";
  }
}