Not all CSS definitions can be applied to all HTML or XHTML tags. Whether a particular CSS property can be applied (or not) depends on the nature of the tag. For the most part, it's fairly obvious if the property can be applied.
For example, you wouldn't expect the text-indent property, which indents the first line of a paragraph, to apply to an inline tag such as <b>, and it can't be. If you do find you need some help in this area, see this book's companion Web site (webbedenvironments.com/css_dhtml_ajax) for a list of the properties that can be used with a particular kind of HTML or XHTML tag.
HTML and XHTML tags can generally be sorted into three categories: