Preparing content for the web is much more than just copying text from your favourite word processor and pasting it into the framework of a web page. In order to ensure the greatest accessibility and widespread availability of web content, it is important to consider that there is more to the web page than the words and pictures that end up in a person's browser.
Web pages are built with HTML, a structural language that is used to denote the different structural parts of the document. HTML employs an extensive series of opening and closing tags to mark parts of the document as, for example, headings (< h >< /h>), lists (< li >< /li>), quotations (< q >< /q>), paragraphs (< p >< /p>), and abbreviations and acronyms (< abbr >< /abbr> or < acronym >< /acronym>). These tags don't appear in the final view of the web page, but are used by the various web software applications to interpret the intended format of the text for output to the end user.
To better understand this structural logic, consider your word processing application, for example Microsoft Word. When authoring documents you can make headlines by highlighting the text and making it bold and a larger font size. But the better way is to use the style chooser functionality of Word. By doing so, whether you realize it or not, you are imparting structure to your document beyond a visual representation. As a matter of fact, the latest releases of Microsoft Word use a related technology to web authoring (called XML) to attach style to the text. One of the advantages of using the style chooser is that Word can then auto-generate a Table of Contents or other directory. This same construct holds true for web development - by adding the structural logic to your content you are making it easier to convert it to HTML.
To help clarify the term semantic further, consider that some HTML tags (such as the 'b' and 'i' tags, for bold and italic formatting) are not considered semantic elements but are presentational elements, i.e. while they do alter the visual presentation of the text, they don't define the actual structure of the text.
Using semantically structured HTML and then changing the way that the HTML looks with Cascading Style Sheets results in the final version of the web page that looks aesthetically pleasing to people viewing the web page in a browser, and is logically structured, providing meaning to those that need it. Pages built this way are not only accessible to the largest possible audience, but are also well set for future revision of both style and content, and can be easily adapted to accommodate emerging technology and advancing web development standards.