"Let's make habr cleaner ..."
W3C standards , someone listens to them, someone does not, well, but someone does not know what it is and sleeps. I myself sincerely believed and believe that the standards are good, but in moderation, otherwise it sometimes leads to disappointment.
In principle, errors, inaccuracies and other blunders do everything, we are not robots, because this topic about the validity of the habr is more informative, because as a goal to be smart, I, as a habr user, did not pursue, just wanted to provide information for reflection.
Once there is a line in the file header, then excuse me, but you need to adhere to the standard, otherwise the meaning is lost.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Url with test results