Users are increasingly accessing the Internet from information appliances such as PDAs, cell phones, and set-top boxes. Since these devices do not have the same rendering capabilities as desktop computers, it is necessary for Web content to be adapted, or transcoded, for proper presentation on a variety of client devices. In this paper, we propose an annotation-based system for Web content transcoding. First, we introduce a framework of external annotation, in which existing Web documents are associated with content adaptation hints as separate annotation files. We then explain an annotation-based transcoding system with particular focus on the authoring-time integration between a WYSIWYG annotation tool and a transcoding module. Finally, after giving an example of content adaptation using a page fragmentation module for small-screen devices, we compare our approach with related work.