Search engines use complex algorithms to discern among websites (pages) and rank them according relevancy and quality of information. They do this by analyzing HTML code and numerous meta tags. Many other factors taken into account, like page load speed and domain name age, for example. This is why every website needs to have certain design aspects included, and configured properly, in order to get listed, rank well, and receive web traffic from search engines.
A list of things to consider when building or updating a website:
The size (volume) of a website can have a huge impact on search engine rankings. Search engines love content, and they need new content constantly. Larger sites usually rank higher, and adding new content constantly is a great growth strategy.
On the contrary, when it comes to file sizes it's better to keep it small. Large files take lot's of space to store, time and bandwidth to transfer and download, which affects business bottom line and user experience. Always seek to optimize all image files and code.
Websites that use graphics heavily are at SEO disadvantage, because search engine robots cannot read text in graphics or animation. Same is true for pages with small amount of text. Use of graphics and Flash needs to be balanced with HTML to be available to search engines spiders and visitors. And use of ALT tags for images and photos is a must.
Dynamic Web Pages
Database based websites (such as e-commerce or CMS sites) may get URLs that are generated on demand and consist of very long strings of random letters, numbers and alphabet characters. In most cases, search engines will ignore such URLs.
One of the solutions to this problem is to use mod_rewrite command to create spider-friendly URLs. A detailed site map with static links going to and from the dynamic pages, with a good internal linking system may help, as well as having part of your site's content written in HTML.
Proper Use of HTML
The truth about HTML is — not all HTML source code is created equal; writing/reading HTML is alien to the majority of humans, and most of the HTML writing software is seriously flawed. Another truth is that websites with well-written code achieve higher rankings.
The best written HTML is hand-written HTML, and word processors create HTML documents with bloated code which the search engine robots can not process efficiently. A good choice for web design software is Macromedia Dreamweaver, it is versatile and is the industry standard.
Choosing a Domain Name
SEO starts right from choosing a domain name. Ideally, your primary keyword(s) should be your domain name, or included in your domain name. Best domain names are common words, or combinations of words, that are short and easy to remember, descriptive of your business, or include your name. Since almost all common words are taken, using dashes in domain name addresses is acceptable.
Top level domain extensions .com, .net, .org are preferable, but depending on your site's purpose other extensions like .us or .mobi are fine.
Don't use frames because search engines don't like them, some visitors can not see them, and they wreck havoc in SEO. Do use PHP and CSS instead.
Keep It Up To Date
Search engines, just like people, don't like old news. For any website to grow, and keep good ranking, it is absolutely essential to stay up to date. In fact, websites that constantly update experience higher rankings in search engines.