@SebastianPatten - what's wrong with validate.w3.org? Sometimes it happens that you misplace some tags around(especially easy to do with dynamic websites) and the validator can easily point out such issues. Modern browsers for the most part seem to be pretty smart about such issues, but it doesn't hurt to have as valid of an HTML as possible :)
@SebastianPatten I rarely validate HTML anymore, I just make sure open and close tags are matched. After a while, good and proper syntax becomes second nature.
According to the W3 specs, <link> tags are only supposed to go in the <head> section:
Back in 2013, if you put a link tag within the body of the HTML document, it would not validate using validate.w3.org with rules based on HTML 4.01.
For the moment, one could add a link to a stylesheet within the body, but it is not clear what the advantages are.
I don't see any validation errors for using <link> outside of <head>.
If you read the MDN documentation for the link entry (MDN Link entry), you see that if the link element has an itemprop attribute, then the link can appear in flow and phrasing content, thus, in the body.
On a first reading, the HTML 5.0 specification document seems to imply that link's should appear only in the head element of the document. However, if you validate using a HTML 5.0 validator, then the documents appears okay even if you have a link in the flow content.
The best explanation for this discrepancy may be as follows.
The itemprop is part of the microdata specification and is relatively new (read about HTML Microdata) and it is worth reading.
This may be the reason why HTML 5.0 validators do not issue a warning even if the itemprop attribute is not present.