Do you use DOCTYPE without any reference?
  v1.0 Posted at 16/12/2014 5:02 AM by Tiago Araujo
Since HTML5, DOCTYPE no longer requires a reference to a DTD. Back in HTML 4.01, The DTD links were used in to specify the rules for the markup language (Transitional, Strict, Frameset etc) so that the browsers render the content correctly. It’s no longer necessary.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

Figure: Bad Example – XXX

<!DOCTYPE html>

Figure: Good Example – XXX

For more information, go to HTML !DOCTYPE Declaration on w3schools.com​​

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