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chromium/third_party/blink/web_tests/fast/parser/compatMode-in-xhtml.xhtml 

<!DOCTYPE html> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
 <head> 
  <meta charset="utf-8" /> 
  <title>XHTML</title> 
 </head> 
 <body> 
 <script>
    if (window.testRunner)
        testRunner.dumpAsText();
    alert(document.compatMode);
 </script>
 </body> 
</html>