This file contains <center;> ... </center;>

The 'experimental' Nu Markup Checker flags this as an HTML5 ERROR with the string "Error: The center element is obsolete. Use CSS instead."

If 'clean' at present tidy 'silently' replaces this with a <div class="c1">, and add the CSS in TY_(CleanDocument)(doc), ...

Q: Should this behaviour be modified?

A: There are various possibilities

• If the document is configured (or found?) to be HTML5, then
  • If clean configured
    • Fix it silently, as now
    • Fix it with a warning
    • No fix and flag as an error, like the validator
  • Clean not configured
    • No fix, with no warning or error, as now
    • Fix it with a warning
    • No fix and flag as an error, like the validator
• Else if NOT configured (or found?) to be HTML5
  • If clean configured
    • Fix it silently, as now
    • Fix it with a warning
  • Clean not configured
    • Flag as a warning
    • Flag as an error
    • No warning or error, as now

Also perhaps Tidy5 needs another configuration parameter as suggested by the validator.w3.org, namely "HTML4-specific tokenization errors are enabled." That is being able to clearly inform tidy that is the document is to be taken as HTML5, then certain tags are to be an error.