Attributes are meant to be added to a tag in order to define a tag. Attributes consist of two parts which are attribute name and attribute value. Image source is meant to define a relative location on a server where as an alternative text is added as a description. These attributes are important for the structure of a document.
CSS is a mechanism used for studying XHTML which means you cant start with it. XHTML has two sets of tags, one in the XHTML and a second in the DTD. This makes the code XML Complient and allows the XHTML to be transcended by the limitations of HTML allowing it to expand over time.
When starting the development process you should start by thinking about the structure and content. Making up a design and getting the approval of the client before continuing is important. Focus should be on what the most meaningful tag to wrap around each piece of content will be.