title: web in xhtml strict but use html5 attr
body: <p>I read part of the code which webERP's PHP layout.</p>
  
  <p>It use the xhtml1-strict DTD. Then the placeholder appear</p>
  
  <pre><code>&lt;input ... placeholder="..." /&gt;
  </code></pre>
  
  <p>I wonder if the smart web browser will do the xhtml and accept the html5 attr.</p>
  
  <p>What would be different whlie we define a DTD to validate it, and enjoy the advantage of strict programming style ?</p>