Consider output of, e.g., http://validator.nu/?=&doc=http://dev.w3.org/html5/tests/validation/full/invalid/unknown-attribute/link.html

In that output, the following is the real error message:
[[
Attribute bar not allowed on element link at this point.
From line 5, column 1; to line 5, column 44
e</title>↩<link bar="foo" href="url" rel="stylesheet">↩</hea
]]

But also in that message, non-error informational details are included (details scraped from the spec):
[[
Element-specific attributes for element link:
href
rel
media
hreflang
type
sizes
Also, the title attribute has special semantics on this element.
]]

Ideally, it seems like the actual error-message part should be separated/separable from the informational part. I can imagine, for example, that some users might want to suppress the informational content and just see the actual errors. Currently, they can't do that for this case.