When working within blog templates and individual posts (as well as both the Blogger and Haloscan commenting systems, among others), you will likely use a lot of physical markup in your HTML. You might not know it's called physical markup, but whenever you surround text with a tag pair intended to alter the appearance of content rather than the meaning of the content, you're using a physical style.
For instance, think about the tag pair for bolded text: <b> and </b>. You use this bit of physical markup specifically because you want that text to be bold, not because you want it to represent a specific level within the overall content hierarchy. Other elements of physical markup include the <i></i> tag pair for italicized text, <tt></tt> for typewriter text, <u></u> for underlined text, and <s></s> for strikethrough text. You can define the standard physical markup tags any way you'd like, via your stylesheet. Suppose that you want all text surrounded by the <b></b> tag pair to have a font-weight attribute of bold but also to appear colored red, regardless of the other text colors used in your document. You can style the bold tag to always be red, just as you can style the typewriter text tag to always appear in the Courier New font, and so on. You will learn numerous options for styling items in Appendix B.