According to Dictionary.com, "semantics refers to the correct interpretation of the meaning of a word or sentence." Using a word semantically is to line it up with the proper meaning of the word. The misuse of a word is not using that word semantically correct. Most tags that are HTML have semantic meaning to them. This means that the element that is written has information about the content that is in between the opening and closing tags. An example of this is when a browser runs into a h1 heading, and it clarifies that heading as the most important heading on that page.
When writing semantic Markup, HTML tags are used to let browsers know something about is contained in the element. Nowadays tags have become a way to tell any machine something about the meaning of the content. In order to write semantic markup people must use HTML tags correctly so that the markup is human and machine readable.
To an average website, good CSS can make bad markup invisible. There is no amount of styling that will make bad markup more meaningful to a computerized visitor such as a search engine web crawler. According to Bruce Lawson, semantic use of HTML "enhances accessibility, searchability, internalization, and interoperability. Writing semantic markup is mandatory if you want your website to be accessible to all visitors, and to be available to visitors from around the world. When the web can read both humans and computers, it becomes more accessible since computers are better able to analyze its contents.