The warning “Using experimental feature: HTML5 Conformance Checker.” means that the validator is checking your markup as HTML5. It doesn’t mean that there is anything wrong with the code. But W3C hasn’t accepted HTML5 as a standard, so validator provides a warning for that, because HTML5 standard is not complete yet. So it is recommended to validating with doctype of “HTML 4.01 Strict”.
For more information about Recommended list of Doctype declarations, visit this link.