184 |   | ::-moz-focus-inner is an unknown vendor extended pseudo-element
184 |   | ::-moz-focus-inner is an unknown vendor extended pseudo-element
184 |   | ::-moz-focus-inner is an unknown vendor extended pseudo-element
184 |   | ::-focus-inner is an unknown vendor extended pseudo-element
192 |   | :-moz-focusing is an unknown vendor extended pseudo-class
192 |   | :-moz-focusing is an unknown vendor extended pseudo-class
192 |   | :-moz-focusing is an unknown vendor extended pseudo-class
192 |   | :-moz-focusing is an unknown vendor extended pseudo-class
248 |   | ::-webkit-inner-spin-button is an unknown vendor extended pseudo-element
248 |   | ::-webkit-outer-spin-button is an unknown vendor extended pseudo-element
266 |   | ::-webkit-search-decoration is an unknown vendor extended pseudo-element
276 |   | ::-webkit-file-upload-button is an unknown vendor extended pseudo-element

I have gone through the normalize CSS, I have managed to eliminate the errors, but the pseudo-classes are proving awkward. If MDN lists the pseudo-classes, as well as CSS3, why does the validator flag them up?

If I have left the 'normalize.css v8.0.1 | MIT Licence | github.com/necolas/normalize.css' at the top of the stylesheet, is that sufficient to cite and avoid plagiarism?