Answers.Body: <p>I believe you are confusing ePub2.0 vs ePub 3.0.  If I recall ePub 4 is for Google only.  If you are worried about older devices I would focus on validating your <code>.epub</code> file with a <a href="http://validator.idpf.org/" rel="nofollow">2.0 validation tool</a>.</p>
  
  <p>To answer your question if its backwards, yes, <strong>only if it validates across the board</strong>:</p>
  
  <p><strong>Example:</strong></p>
  
  <pre><code>&lt;File&gt;
      &lt;Name&gt;foo.epub&lt;/Name&gt;
      &lt;ValidationReport elapsedTimeInMillis="230"&gt;
          &lt;Version&gt;3.0.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="177"&gt;
          &lt;Version&gt;1.2.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="149"&gt;
          &lt;Version&gt;1.1.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="156"&gt;
          &lt;Version&gt;1.0.5&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="201"&gt;
          &lt;Version&gt;1.0.4&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="182"&gt;
          &lt;Version&gt;1.0.3&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="193"&gt;
          &lt;Version&gt;1.0.2&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="208"&gt;
          &lt;Version&gt;1.0.1&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
  &lt;/File&gt;
  </code></pre>
  
  <p>The reason is because an <code>.epub</code> file can be accepted by some as a 3.0 valid ePub <strong>ONLY</strong> and it could still fail the ePub 2.0 validation.  </p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2013-12-19T23:16:42.730Z
Answers.LastActivityDate: 2013-12-19T23:22:28.377Z
Answers.LastEditDate: 2013-12-19T23:22:28.377Z
Answers.UserDisplayName: DᴀʀᴛʜVᴀᴅᴇʀ
Body: <p>I am planning to publish a book and I am wondering whether to distribute it as ePub3 or ePub4. My concern being that older eBook readers may still be out there and I do not wish to exclude them. I would prefer ePub3 if the older readers are able to read it.</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2013-12-19T22:49:34.370Z
Comments.Text: It is unclear what you mean: are you asking if older readers support ePub4 or if new ones support ePub3?
Comments.UserDisplayName: Ian
Tags: <epub><publishing>
Title: Is ePub3 backward compatible?
UserDisplayName: FlowRaja
ThreadText: # Is ePub3 backward compatible?
  
  (Asked by: username_0 on 2013-12-19)
  
  I am planning to publish a book and I am wondering whether to distribute it as ePub3 or ePub4. My concern being that older eBook readers may still be out there and I do not wish to exclude them. I would prefer ePub3 if the older readers are able to read it.
  
  **Tags:** epub, publishing
  
  > **Comments:**
  > **username_1**: It is unclear what you mean: are you asking if older readers support ePub4 or if new ones support ePub3?  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_2 on 2013-12-19. Score: 4 | Accepted answer
  
  I believe you are confusing ePub2.0 vs ePub 3.0.  If I recall ePub 4 is for Google only.  If you are worried about older devices I would focus on validating your .epub file with a 2.0 validation tool.
  To answer your question if its backwards, yes, only if it validates across the board:
  Example:
  
      foo.epub
      
          3.0.0
          yes
          
          
      
      
          1.2.0
          yes
          
          
      
      
          1.1.0
          yes
          
          
      
      
          1.0.5
          yes
          
          
      
      
          1.0.4
          yes
          
          
      
      
          1.0.3
          yes
          
          
      
      
          1.0.2
          yes
          
          
      
      
          1.0.1
          yes
          
          
      
  
  
  The reason is because an .epub file can be accepted by some as a 3.0 valid ePub ONLY and it could still fail the ePub 2.0 validation.
  
  
  ---
  
  