iframes are a serious issue regarding SEO... the content is NOT visible to search engines! So, in my opinion they should never be used. If your webdsite can not achieve good ranking and be found easily by viewers, it is a wasted effort. Designing with tables, except for tabular data, is also not very SEO friendly, or the best choice. Clean CSS coding is the way to go. It allows beautiful design, less code and is SEO friendly.