Ｗ３Ｃの検証サービスで検証をしていて ↓のようなメッセージが出ていた場合

Sorry, this type of URL scheme (undefined) is not supported by
this service. Please check that you entered the URL correctly.
URLs should be in the form: http://validator.w3.org/
If you entered a valid URL using a scheme that we should support,
please let us know as outlined on our Feedback page.
Make sure to include the specific URL you would like us to support,
and if possible provide a reference to the relevant standards document
describing the URL scheme in question.
Remember that you can always save the page to disk
and Validate it using the File Upload interface.

[解決方法]

「Validate by URL」のところから、「Upload」してみてください。