title: window.atob fails: "string to be decoded is not correctly encoded" on strings of specific lengths
body: <p>Tried <code>window.atob(x);</code> where <code>x</code> is <code>"A"</code>, <code>"AA"</code>, <code>"AAA"</code>, and so forth.
  Why does the browser say:</p>
  
  <blockquote>
    <p>VM156:1 Uncaught DOMException: Failed to execute 'atob' on 'Window':
    The string to be decoded is not correctly encoded.</p>
  </blockquote>
  
  <p>when the string length is 1, 5, 9, 13, 17, ...</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>["A","AA","AAA","AAAA","AAAAA","AAAAAA","AAAAAAA"].forEach(function(str) {
    console.log(str.length,str);
    try {
      console.log(atob(str));
    }
    catch(e) {
      console.log(e.message);
    }
  });</code></pre>
  </div>
  </div>
  </p>