I'm using a slash whenever the URL doesn't end in a file extension. Because that's the right way to do it. We can also stop putting dots at the end of sentences
You don't use a full URL in code. The general principle is to be flexible and to use shortcuts and clever solutions whenever possible. Although, pinkytoe's scraping argument is a really good one. But Christiaan's http/https argument beats that one and ends the discussion in the relative URL's favor.
There are no SEO advantages either way.
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