Search engines are limited in how they crawl the web and interpret content. A website doesn’t always look the same to you and me as it looks to a search engine. In this section, we’ll focus on specific technical aspects of building (or modifying) web pages so they are structured for both search engines and human visitors alike. Share this part of the guide with your programmers, information architects, and designers, so that all parties involved in a site’s construction are on the same page.
To perform better in search engine listings, your most important content should be in HTML text format. Images, Flash files, Java applets, and other non-text content are often ignored or devalued by search engine crawlers, despite advances in crawling technology. The easiest way to ensure that the words and phrases you display to your visitors are visible to search engines is to place them in the HTML text on the page.
Seeing your site as the search engines do
Many websites have significant problems with indexable content, so double-checking is worthwhile. By using tools like Google’s cache, SEO-browser.com, and the MozBar you can see what elements of your content are visible and indexable to the engines. Take a look at Google’s text cache of this page you are reading now. See how different it looks?
Crawlable Link Structures
Just as search engines need to see content in order to list pages in their massive keyword-based indexes, they also need to see links in order to find the content in the first place. A crawlable link structure—one that lets the crawlers browse the pathways of a website—is vital to them finding all of the pages on a website. Hundreds of thousands of sites make the critical mistake of structuring their navigation in ways that search engines cannot access, hindering their ability to get pages listed in the search engines’ indexes.
Below, we’ve illustrated how this problem can happen:
In the example above, Google’s crawler has reached page A and sees links to pages B and E. However, even though C and D might be important pages on the site, the crawler has no way to reach them (or even know they exist). This is because no direct, crawlable links point pages C and D. As far as Google can see, they don’t exist! Great content, good keyword targeting, and smart marketing won’t make any difference if the crawlers can’t reach your pages in the first place…
article by moz.com | the basics of search engine friendly design & development