We could spend some time here and spout off some of the standard wisdom about what search engines uses what rules to index documents. But in my experience, this information is generally unreliable.
Most search engines are very secretive about the indexing rules they use.
Some will publish one rule and use another. Some change their rules all the time. The only reliable way to figure out how each search engine works at any particular time is to compare search results. Using this method, you can figure out what a search engine is looking for at this particular time. Modify your pages to accommodate this and then submit.