Simon Willison’s Weblog

We did some studies and found that the attribute was almost never used, and most of the time, when it was used, it was a typo where someone meant to write rel="" but wrote rev="". To be precise, the most commonly used value was rev="made", which is equivalent to rel="author" and thus was not a convincing use case. The second most common value was rev="stylesheet", which is meaningless and obviously meant to be rel="stylesheet".

— Ian Hickson