Add default tab using javascript

    Buttons "next" and "previous" now change the active tab using javascript

canvas.js

@@ -74,7 +74,6 @@ for (let i = 0; i < bgColorElems.length; ++i) {
 }
 document.getElementById('step2value').oninput = draw;
 document.getElementById('signature').oninput = draw;
-window.onload = draw;
 
 let navElems = document.querySelectorAll('nav > a');
 for (let i = 0; i < navElems.length; ++i) {
@@ -85,3 +84,16 @@ for (let i = 0; i < navElems.length; ++i) {
     e.target.classList.add('active-tab');
   }
 }
+
+let navButtons = document.querySelectorAll('.prevButton, .nextButton');
+for (let i = 0; i < navButtons.length; ++i) {
+  navButtons[i].onclick = function(e) {
+    document.querySelector('nav > a[href="' + e.target.attributes.href.value + '"]').click();
+  }
+}
+
+window.onload = function() {
+  draw();
+  let hash = window.location.hash || '#step1';
+  document.querySelector('nav > a[href="' + hash + '"]').click();
+}