Semantic Markup is the use of HTML markup to reinforce the semantics or meaning of the information in webpages and web applications rather than to define its presentation or look. In an HTML document, author may "start with a title"; add headings and paragraphs and add emphasis to texts; add images or links to other pages and various kinds of lists.
Search Engine Optimization is the process of increasing visability of a website or a webpage to the user of the web search engine. SEO is performed becuase it can result in more visitors from the search engine and further more these increase in visitors can be converted into customers. Optimizing SEO can be done through efficientaly using the semantic markup of the webpage done through modifying the HTML and associated coding to increase its relevance to specific keyworks and remove barriers to indexing activities of search engine. In 2013 Google Hummingbird update featured an algrothim change to improve Google's natural language processing system and semantic understanding making semantic markup more important to getting your website indexed on the search engine more efficientaly.
Section 508 of the Rehabilitation Act ties into semantic markup. this federal law states that all electronic and information system created needed to easily accessible to people with disabilities. People suffering from disabilities such as bad eye sight can have machines that read out content on a webpage. This semantics play a large role in the accessability of information.
Semantic markup and haveing a seperate style sheet area crucial for making content on websites understandable by blind people, search engines, and everyone else.
Semantically marked up content is read in the order that appears in the document. Website creators can style it to be attractive to indiviudal with good vision and also make websites more appealing to individuals with sensitive eyes. Search engines are not able to "see" what is exactly on the pages and semantic markups help them index the content to lead people to the page they want to see.