Systems and methods are provided for determining customized markup content to deter malicious attackers and/or to decrease electronic submissions from robots. In some embodiments, markup content may be randomized with unique identifiers, reordering of markup elements, and/or insertion of hidden markup elements. The modifications to markup content may have no impact on human usability of the markup content. However, the customized markup content may render the content unusable by a programmed, automated attacker that cannot parse and/or recognize the content. Thus, automated attackers are deterred from using markup content, while human users remain unaffected.