title: Wait for document.body existence
body: <p>I wrote a Chrome extension that works before the page is loaded (using the attribute <code>&quot;run_at&quot;: &quot;document_start&quot;</code>).
  The problem is that I want to add a <code>div</code> tag to the web page body as soon as it is created.
  Before that <code>document.body</code> is null so I can't append tags to it.</p>
  <p>I don't care about full load of the body, I just need it to be existent.</p>
  <p>I am trying to find the best way to be alerted when the <code>body</code> tag in html is created (not loaded fully, just created). Is there any event handler for this case that I can write?</p>
  <p>Also, I don't want to use <code>jQuery</code> or any other non built-in library.</p>