Alt Tag – Most website owners know about this, but including an Alt Tag on your images is actually required if you want to have a properly formed website.
<acronym> - The acronym tag allows for a website to explain what an acronym stands for. For example, the acronym SEO stands for Search Engine Optimization. However, when the spider comes to your website, all it will see is SEO which may mean various things. The acronym tag will allow you to add this keyword to your text. The use of this tag should look like this: <acronym title="search engine optimization">seo</acronym>
<caption> - Although using tables to determine the layout of your website is becoming a practice that will soon be extinct, tables will still be necessary. The caption tag allows you to identify what a specific table is about. An example: <caption>Table 3.2. Raw Sales Data</caption>
i took this from a long artical on SEO Through
Well Built Pages
By Mark Daoust
Full On Design