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<style> or <link> without a type attribute is fine. No reason I "should add" one.
To be clear, HTML5 makes the type attribute optional For reference: http://dev.w3.org/html5/spec/the-link-element.html#attr-link-type http://dev.w3.org/html5/spec/the-style-element.html#attr-style-type And browsers handle link and style content just fine if the type attribute is omitted. So for authors there really is not much point in including it and for validators there is not much point in checking for it or reporting anything about it.
In particular, this seems rather out of scope for a *CSS* validator.