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Today there are two primary standards that are used as benchmarks for Web Accessibility.
WCAG is produced and maintained by the World Wide Web consortium (W3C). WCAG's standards are based on three levels named Priority 1, Priority 2, and Priority 3. Priority 1 maintains the most basic requirements that a site should attain in order to be accessible. A good place to start in getting more information about WCAG can be found in their Web Content Accessibility Guidelines 2.0
Section 508 is a US federal law enacted in 1998. Its basis was for use in all government agencies but it has since been adopted by the wider community as a benchmark for web accessibility. For more information on Section 508 we suggest you have a look at the official website.