Generated HTML should be valid XML (XHTML)

[icnocop]

No description provided.

[filzrev]

Currently docfx output HTML files that based on HTML Living Standard(HTML5)
So I thought default HTMLPostProcessor behaviors should not be changed with following reasons.

• If HTMLPostProcessor don't change file content. It can optimize performance by skipping overwrite files ([Feature Request] Don't overwrite output files when output file is not changed. #8964)

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What's about adding custom XHtmlPostProcessor and specify custom postprocessor with docfx.json?

"build": { 
  "postProcessors": ["XHtmlPostProcessor"]
}

[icnocop]

Thank you @filzrev !
It worked and added the code for the solution to #10191