Most Likely Cause and Fix
Your HTML5 document contains an invalid link relation in
the context of
<a rel="category tag" … />.
rel is not allowed to take the value
What This Error Means
Some element attributes can take pretty much any value, others can take only values of a certain type and others still can only take a value from a pre-defined set.
This error relates to the following usage in your markup:
<a rel="category tag">
The error is stating that, in this context, the attribute
a is not allowed to take the value
The validator is giving the reason: the string category is not a registered keyword or absolute url.