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Why can't I use the form tag in the store customization box at CafePress?
The store customization page at Cafepress.com displays the customized store "header" inside of a form tag. The HTML specification does not support nested form tags (forms inside forms), so the result of this is that the customization page ends up being garbled. A similar thing will happen with certain types of HTML errors; you can wreck the customization page if you forget to put a closing quote on an href in a link tag, for example.
Although it could hardly be recommended, you could be very careful that your custom HTML was exactly right and include the form tag. You'd never be able to customize your store again, but the form tags would end up on your storefront. (Matthew from The ARMAC Store)