Finding a consistent location for items that do not display, such as style sheets, symbol libraries, fonts (or other encoded CDATA), and animation elements, can tremendously ease time spent editing a document. By ensuring future editors of your code (this includes you) a dependable location to find certain elements, valuable time can be spent modifying code, not searching for it.
This book's examples utilize a consistent method of organizing document elements. This method organizes content in the following order:
Document data (<?xml>, <!DOCTYPE>, and so forth)
Symbol libraries (<defs>, <symbol>, and so forth)
Document content (<rect>, <path>, and so forth)
Animation and interaction ...