21st Century Integrated Digital Experience Act or the 21st Century IDEA (Sec. 3) This bill requires an executive agency that creates a public website or digital service, or that redesigns an existing public legacy website or digital service, to ensure that the website or service: is accessible to individuals with disabilities, has a consistent appearance, does not overlap with or duplicate any legacy websites; contains a search function, is provided through an industry standard secure connection, is designed around user needs, provides users with the option for a more customized digital experience that allows users to complete digital transactions in an efficient and accurate manner, and is fully functional and usable on common mobile devices. Each agency that maintains a public website or digital service shall report: (1) to Congress on those websites and services, including the cost of modernization, and (2) annually to the Office of Management and Budget (OMB) on the agency's progress in implementing the requirements. Agencies must ensure that Intranets established after enactment of this bill conform to the requirements for public websites. Any website of an agency that is made available to the public after enactment shall be in compliance with the website standards of the Technology Transformation Services of the General Services Administration (GSA). (Sec. 4) The OMB shall issue guidance to each agency to: (1) identify public nondigital, paper-based, or in person government services; and (2) include in the agency's budget request a list of nondigital services with the greatest impact that could be made available to the public through an online, mobile-friendly, digital service option in a manner that decreases cost, increases digital conversion rates, and improves customer experience, and an estimation of the cost and schedule associated with carrying out such modernization. Agencies must make forms used by the public available digitally and report on forms, services, and processes that cannot be digitized. Each agency shall maintain an accessible method of completing digital services through in-person, paper-based, or other means, such that individuals without the ability to use digital services are not deprived of, or impeded in, access to such services. (Sec. 5) Each agency shall submit to the OMB and Congress a plan to accelerate the use of electronic signatures. (Sec. 6) Each agency's Chief Information Officer shall: coordinate and ensure alignment of the internal and external customer experience programs and strategy of the agency, coordinate with the management leaders of the agency to ensure proper funding to support this bill's implementation, continually examine the agency's public digital service delivery strategy and submit recommendations providing guidance and best practices, identify areas of concern that need improvement using data relating to customer experience and satisfaction, and coordinate and ensure compliance with requirements for federal websites to be mobile friendly. (Sec. 7) Each agency shall: (1) seek to maintain as much standardization and commonality with other agencies as practicable in implementing this bill's requirements to best enable future transitions to centralized shared services; and (2) coordinate the implementation of such requirements, including the development of standards and commonalities. The GSA must make available under a Federal Supply Schedule the systems and services necessary to fulfill this bill's requirements. Such schedule shall ensure interoperability between agencies, compliance with industry standards, and adherence to best practices for design, accessibility, and information security.