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They're using microformats, specifically hCard and hCalendar. Along with RDFa and JSON-LD, this is an alternative to microdata. See Google's Rich Snippet spec for people here, and here's my public LinkedIn page viewed with Google's structured data testing tool, showing a preview Rich Snippet and the extracted structured data.
Max, Google does support structured data for Organization markups, but they don't support rich snippets for the type. If you change schema.org/Organization to schema.org/LocalBusiness in your markup, you'll see the testing tool results change and the rich snippets displaying. So the error message you're seeing is in regards to rich snippets, not the ...
Although their documentation doesn't say so, using the rel attribute is supported by Google, so both should work. The 'in-URL' ?rel= syntax exists for blog users, who may not have access to edit the page's HTML tags, but the HTML5 separate attribute version is more 'correct' so I'd recommend: <a href="[profile_url]" rel="author">Author</a>
This depends on the actual content (there is an important difference between, e.g., a hash and ASCII art) and the context (editable vs. presented content). So after choosing the appropriate element (and possibly WAI-ARIA), HTML5 offers the following options: The language tag zxx (IETF BCP 47/IANA registry) can be used for "no linguistic content": ...
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