W3C CSS Validator errors i don't know how to solve.

Hi there, i am trying to improve my CSS, and at the moment i am trying to style a form that is using HTML5 validation (putting "required" in the tags and stuff like that) to make sure that the user don't forget to write his or hers name or something. It works really good, but i like to validate my code to see if something is wrong, and then fix it (I have learned a lot this way!). I haven't been able to come up with a solution here though, and hopefully i can get some info from someone here on the Treehouse community!

This is the error shown when validating:

Unknown pseudo-element or pseudo-class :required Unknown pseudo-element or pseudo-class :invalid Unknown pseudo-element or pseudo-class :valid

I am using them like this:

input:required {
    border: solid 1px black;
}

input:invalid {
  border: solid 1px red;
}

input:valid {
    border: solid 1px green;
}

Thanks in advance!