Most Likely Cause and Fix
Your markup contains a <pre> element with
a rel attribute in the form <pre rel="value" />.
Attribute rel is not allowed on element <pre>.
You should remove this attribute for your markup to be valid or consider using HTML5 custom data attributes.
What This Error Means
There is a defined set of attributes for each HTML element. You may choose to omit some but you cannot choose to add further attributes beyond those defined.
<img src="http://example.com/image.png" alt="Desc" />
This error is saying that in the context of <pre rel="value" />,
the attribute rel is not allowed.
How To Fix It
In general
The specific error message you encountered will refer to a line and column number. This will guide you to the exact point in your markup to which the error relates.
Your markup contains a <pre> element with
a rel attribute in the form <rel pre="value" />.
Attribute rel is not allowed on element <pre>.
Use
<pre … />
instead of
<pre rel="value" … />
or consider using a HTML5 custom data attribute
<pre data-rel="value" … />