W3 validator: "Attribute xmlns:XX is not allowed here" errors [#2748787]

Testing my Bootstrap themes websites on https://validator.w3.org returns many errors like this:

Attribute xmlns:og not allowed here.
From line 1, column 16; to line 16, column 48
TYPE html>↩<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" version="XHTML+RDFa 1.0" dir="ltr"↩  xmlns:og="http://ogp.me/ns#"↩  xmlns:article="http://ogp.me/ns/article#"↩  xmlns:book="http://ogp.me/ns/book#"↩  xmlns:profile="http://ogp.me/ns/profile#"↩  xmlns:video="http://ogp.me/ns/video#"↩  xmlns:product="http://ogp.me/ns/product#"↩

the list continues with the same error for xmlns:article/book/profile/video/product/content/dc/etc...