Semantic markup can be viewed as getting a return on your investment via web content. Semantic Markup is the use of HTML elements and tools to reinforce the semantics/meaning clearly through content in webpages. An example of this would be a “list tag” to list out items instead of listing them off within a paragraph.
The 508 Accessibility Act of 1973 is a compliance standard act for government agencies, non-profits, and educational institutions that require the websites they create to be accessible to all users regardless of disabilities.
SEO (Search Engine Optimization) is a marketing practice where websites change their markup for search engine optimization. Sites like google and yahoo give higher priority to sites whose markup is defined a certain way. An example of search engine optimization might be to include specific meta tags to your html markup to increase the likelihood of appearing higher up on google.
Semantic markups, 508 accessibility, and search engine optimization are all related due to the fact that all three have to be taken into account when developing websites. Since all 3 use the same markup the goal of a web developer needs to be is to make web pages that satisfy all three criteria’s.