Search engine spiders are simple creatures. While they will visit your Web site and capture every element, there are many page elements they they cannot interpret. Therefore, if your targeted keyword phrases are contained in any of these elements, you should consider moving them into other parts of the HTML so that they will have a chance to be read by the search engine's spider and ultimately returned by the search engines in response to a keyword query. In general, if you cannot highlight it on the page, the search engine cannot read it.