Our forms were designed to adhere as closely as possible to the W3C standards and validate under an XHTML 1.0 Strict doctype.
That said, there are a few issues that would prevent a form from validating 100% under the Strict doctype. Address these issues to be sure your HTML validates properly.
If you select the "Raw HTML Version" when getting the HTML to publish your form, move the form styles, as well as everything between and including the style tags, to the head of your page.
When setting up your thank you page, if you select the "Open Thank You Page in New Window" option, your form will validate as XHTML 1.0 Transitional, but not Strict.