Mif2Go can produce many varieties of HTML, including some that are intended for use with older browsers such as Netscape Navigator 4.x. Some default settings allow such back-compatible code generation, which does not validate to HTML 4 specifications. Also, one particular HTML tag is not accepted by W3C, certain table anomalies in your FrameMaker document can cause validation errors, and several high-ASCII characters cause validator warnings.
If you require clean validation for your output, you might have to make some adjustments to your FrameMaker document and to your configuration settings.