html validation error

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Contact Us - DigiMag</title>
<link href="CSS/external.css" rel="stylesheet" type="text/css">
<style>
body {
  background-color: #90FF70;
}

body {margin:0;}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: #1DE06E;
  position: -webkit-sticky; /* Safari */
  position: sticky;
  top: 0;
}

li {
  float:left;
}

li a {
  display: block;
  color: black;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

li a:hover:not(.active) {
  background-color: #0066E0;
}
li {
  float: left;
  border-right:1px solid #62f5f5;
}
.active {
  background-color: #90FF70;
}
<meta name="viewport" content="width=device-width, initial-scale=1">

body {
  font-family: "Arial, sans-serif";
  box-sizing: "border-box";}

input[type=text], select, textarea {
  width: 100%;
  padding: 12px;
  border: 1px solid #ccc;
  border-radius: 4px;
  box-sizing: border-box;
  margin-top: 6px;
  margin-bottom: 16px;
  resize: vertical;
}

input[type=submit] {
  background-color: #4CAF50;
  color: white;
  padding: 12px 20px;
  border: none;
  border-radius: 4px;
  cursor: pointer;
}

input[type=submit]:hover {
  background-color: #1de06e;
}

.container {
  border-radius: 5px;
  background-color: #62f5f5;
  padding: 20px;
}

</style>
</head>

<body>
<a href="images/Web-Banner.jpg">
<div><img src="Images/Web-Banner.jpg" alt="Web Banner" class="responsive"/></div>
</a>
<div class="navbar">
<ul>
  <li style="font-family: Arial, sans-serif;"><a href="index.html">Home</a></li>
  <li style="font-family: Arial, sans-serif;"><a href="latest.html">Latest</a></li>
  <li style="font-family: Arial, sans-serif;"><a href="about.html">About DigiMag</a></li>
  <li style="font-family: Arial, sans-serif;"><a href="pricing.html">Pricing</a></li>
  <li style="float:right; font-family: Arial, sans-serif;"><a href="login.html">Login</a></li>
  <li style="float:right; font-family: Arial, sans-serif;"><a class="active" href="contact.html">Contact</a></li>
</ul>

<h1 style="font-family: Arial, sans-serif;">If you would like to contact us via email then please <a href="mailto:customersupport@digimag.com">click here.</a><br> If you don't then fill in the form below. Thank You.</h1>

<h3 style="font-family: Arial, sans-serif;">Contact Form</h3>

<div class="container">
  <form action="thanks.html">
    <label for="fname" style="font-family: Arial, sans-serif;">First Name</label>
    <input type="text" id="fname" name="firstname" placeholder="Your name..">

    <label for="lname" style="font-family: Arial, sans-serif;">Last Name</label>
    <input type="text" id="lname" name="lastname" placeholder="Your last name..">

    <label for="country" style="font-family: Arial, sans-serif;">Country</label>
    <select id="country" name="country">
      <option value="UK">UK</option>
      <option value="USA">USA</option>
      <option value="Austraila">Australia</option>
	  <option value="Other">Other</option>
    </select>

    <label for="subject" style="font-family: Arial, sans-serif;">Message</label>
    <textarea id="subject" name="subject" placeholder="Write something.." style="height:200px; font-family: Arial, sans-serif;"></textarea>

    <input type="submit" value="Submit">
  </form>
</div>
</div>
</body>
</html>