According to Dictionary.com, semantics refers to the correct interpretation of the meaning of a word or sentence. To use a word semantically is to use it in a way that is properly aligned with the meaning of the word. When we misuse a word we are not using it semantically. Many HTML tags have semantic meaning. That is, the element itself conveys some information about the type of content contained between the opening and closing tags.
For example, when a browser encounters an h1 heading it interprets that tag to mean that the contents of the h1 element constitute the most important heading of the section that contains the element. The semantic meaning of an h1 tag is that it is used to identify the most important header of a specific web page or section.
As per wikipedia, Search engine optimization (SEO) is the process of affecting the visibility of a website or a web page in a web search engine's unpaid results—often referred to as "natural", "organic", or "earned" results. In general, the earlier (or higher ranked on the search results page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine's users; these visitors can then be converted into customers.
Accessibility standards for the information technology environment have been created by the Access board and are referenced as Section 508 Standards. These laws apply to electronic and information technologies used by federal agencies. They take into account performance of the technologies and have certain criteria to determine functionality ratings. Section 508 was implemented on December 21, 2000 and continues to recieve updates to this day.
You can not establish any one of the rules, without others coming into the picture. They are all co-related. emantic Markup is required to have Section 508 in place as the semantic markup is a key role in allowing the disabled to navigate a website properly. In the case of search engines, one will not get proper search results without the proper semantic markups placed.