Try to validate the page as a visitor (not logged in) to get a realistic result with less interferance from EPiServer's On Page Edit feature.
Also try to inherit from SimplePage instead of TemplatePage to turn off some EPiServer features.
Another thing to do in web.config under the <system.web> section:
<xhtmlConformance mode="Strict" />
One of the things that this declaration do is make the form tag validate correctly.