TYPO3 Core

Actually the inclusion of stylesheets in the body tag is not allowed by the W3C standards until HTML 5.1 (see [1]: "NOTE: If the rel attribute is used, the element is restricted to the head element."), it becomes more and more popular for page optimization purposes.
Therefore it has been added by the WHATWG to their HTML recommendations [2] already.

1: https://www.w3.org/TR/html51/document-metadata.html#the-link-element
2: https://github.com/whatwg/html/commit/179983e9eb99efe417349a40ebb664bd11668ddd

I close this issue in favor of #86869 that already has a review; please continue the discussion there. Thank you. I've also added there a reference to this issue to keep track of it.