diff --git a/index.html b/index.html
index 449b47e..5f3b7a8 100644
--- a/index.html
+++ b/index.html
@@ -1,44 +1,53 @@
+<!DOCTYPE html>
 <html>
-	<head>
-		<link rel="stylesheet" type="text/css" href="style.css">
-		<link href="https://fonts.googleapis.com/css?family=Open+Sans:600,700" rel="stylesheet">
-		<title>A simple calculator</title>
-	</head>
-	<body>
-		<div id="container">
-			<div id="calculator">
-				<div id="result">
-					<div id="history">
-						<p id="history-value"></p>
-					</div>
-					<div id="output">
-						<p id="output-value"></p>
-					</div>
-				</div>
-				<div id="keyboard">
-					<button class="operator" id="clear">C</button>
-					<button class="operator" id="backspace">CE</button>
-					<button class="operator" id="%">%</button>
-					<button class="operator" id="/">&#247;</button>
-					<button class="number" id="7">7</button>
-					<button class="number" id="8">8</button>
-					<button class="number" id="9">9</button>
-					<button class="operator" id="*">&times;</button>
-					<button class="number" id="4">4</button>
-					<button class="number" id="5">5</button>
-					<button class="number" id="6">6</button>
-					<button class="operator" id="-">-</button>
-					<button class="number" id="1">1</button>
-					<button class="number" id="2">2</button>
-					<button class="number" id="3">3</button>
-					<button class="operator" id="+">+</button>
-					<button class="empty" id="empty"></button>
-					<button class="number" id="0">0</button>
-					<button class="empty" id="empty"></button>
-					<button class="operator" id="=">=</button>
-				</div>
-			</div>
-		</div>
-		<script src="script.js"></script>
-	</body>
-</html>
\ No newline at end of file
+<head>
+  <meta charset="UTF-8">
+  <title>A complex calculator</title>
+  <link rel="stylesheet" type="text/css" href="style.css">
+  <link href="https://fonts.googleapis.com/css?family=Open+Sans:600,700" rel="stylesheet">
+</head>
+<body>
+  <div id="container">
+    <div id="calculator">
+      <div id="result">
+        <div id="history">
+          <p id="history-value"></p>
+        </div>
+        <div id="output">
+          <p id="output-value"></p>
+        </div>
+      </div>
+      <div id="keyboard">
+        <button class="operator" id="clear">C</button>
+        <button class="operator" id="backspace">CE</button>
+        <button class="operator" id="%">%</button>
+        <button class="operator" id="/">&#247;</button>
+        <button class="number" id="7">7</button>
+        <button class="number" id="8">8</button>
+        <button class="number" id="9">9</button>
+        <button class="operator" id="*">&times;</button>
+        <button class="number" id="4">4</button>
+        <button class="number" id="5">5</button>
+        <button class="number" id="6">6</button>
+        <button class="operator" id="-">-</button>
+        <button class="number" id="1">1</button>
+        <button class="number" id="2">2</button>
+        <button class="number" id="3">3</button>
+        <button class="operator" id="+">+</button>
+        <button class="empty" disabled></button>
+        <button class="number" id="0">0</button>
+        <button class="empty" disabled></button>
+        <button class="operator" id="=">=</button>
+        <button class="operator" id=".">.</button>
+        <button class="operator" id="^">^</button>
+        <button class="operator" id="sqrt">&#8730;</button>
+        <button class="operator" id="sin">sin</button>
+        <button class="operator" id="cos">cos</button>
+        <button class="operator" id="tan">tan</button>
+        <button class="operator" id="log">log</button>
+      </div>
+    </div>
+  </div>
+  <script src="script.js"></script>
+</body>
+</html>
