[html5] HTML5 validator behavior issue

[Gmail]

HTML5 specs says:

<quote>
3.2.3.7 The style attribute
...
Note: In particular, using the style attribute to hide and show content, 
or to convey meaning that is otherwise not included in the document, 
is non-conforming. (To hide and show content, use the hidden attribute.)
<quote>

The following HTML fragment

<span style="display:none;">
hidden content
</span>

when submited to HTML5 validator return a valid document.

So, what does means "non-conforming"? or is this a validator fail?

TIA
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