Useless stuff (no pain, but no gain as well)

1. Meta Keywords
   Long time ago the <meta name="keywords" content=""> tag was intended to tell search engine the keywords relevant to this particular page. In modern SEO history search engines download websites and extract relevant keywords from their content, so the Meta K tag is not used for web ranking anymore. Simply forget it, it is useless for SEO.
   
2. Keyword Density
   One of the most overestimated web ranking factors is the keyword density. What is keyword density and why this myth lives so long? The keyword density of each particular word on a page is calculated as follows:
   
   KD = Word_Count / Total_Words * 100%
   
   That is, if a page has 150 words and the word "SEO" is mentioned 24 times on that page, its keyword density would be: 24 / 150 * 100% = 16%
   
   But why this value is useless? Because search engines has evolved and does not count on keyword density anymore, since it is very easily manipulated. There are thousands of factors that search engines consider when calculating the page rank, so why would they need such simple (not to say primitive) way to rank pages as to count the number of times a word appears in the page text? You may hear the keyword density of 6% is the best rate, or keep it within 7% to 10%, or search engines like kw density within 3% to 7% and other bullshit. The truth is...
   
   Search engines like pages written in a natural language. Write for humans, not for search engines! A page can have any keyword density from 0% (no keyword on a page at all) to 100% (a page consisting of only one word) and still rank high.
   
   Well, of course you may want to control the keyword density of your pages, but please consider that there is no good value for this factor. Any value will work if your text is written with a human reader in mind. Why would one still want to check for keyword density if it is not count any more? Because it is a quick and dirty way to estimate the theme of a page. Simply do not overestimate this thing, it is merely a number, nothing more and it is useless for SEO.
   
   Another interesting question: why this myth is still alive and why there are so many people still talking about keyword desnity as an important ranking factor? Perhaps, because keyword density is easy to understand and modify if needed. You can see it right here with your naked eye and quickly learn if your site is going good or bad. Well, it only seems as that, but not actually is - keyword density is useless, remember?
   
3. Dynamic URLs vs. static URLs
   Beleive me or not, there is no difference. Both are of the same SEO value. The days when search engines had difficulties indexing dynamic URL websites are gone for good.
   
4. www.site.com vs. site.com
   No difference either. If you want your site to be accessed with both ways, please add something like this into your .htaccess file:
   
   RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain.com
RewriteRule (.*) http://www.domain.com/$1 [R=301,L]
   
5. Underscore vs. hyphen in URLs
   Once again, there is no any difference from the SEO point. You can use underscore, or hyphen, or even don't use any separator at all - this neither helps, nor hurts your position in SERPs.
   
6. Subfolders
   Is it better to have a /red-small-cheap-widget.php file rather than/widgets/red/small/cheap/index.php? Does it hurt your rank if you put the content deep into the subfolders? The answer is no, it won't hurt your rankings and actually it doesn't matter at all how deep in the folder tree a file is located. What matters is how many clicks it takes to reach that file from the homepage.
   
   If you can reach that file in one click - it certainly is more important and would have more weight than say some other file located within 5 clicks away from the index page. The homepage usually has many link juice to share, so the pages it directly links to are obviously more important than others (well, since they receive more link juice, that is).
   
7. W3C validation
   W3C is World Wide Web Consortium - an international consortium where Member organizations, a full-time staff, and the public work together to develop Web standards. Basically speaking, they are guys who invented HTML, CSS, SOAP, XML and other web technologies.
   
   Validation is the process of checking a page or website for its compliance with W3C standards. You can run a validation of any website for free here. Note, this validator shows not only such trivial things like unclosed quotation, undefined tags or wrong attribute values. It also checks the encoding problems, the compliance with the specified DOCTYPE, obsolete tags and attributes and many more.
   
   Why is validation needed? A 100% valid website ensures that it will display correctly (and identically!) in all browsers that support standards. Unfortunately, in real life some browsers do not strictly follow the W3C standards, so a variety of different cross-browser problems with the number of websites are not rare thing all over the web. This doesn't belittles the importance of W3C standards, however.
   
   From the SEO point the validation doesn't look so crucial though. Run a validation through google.com and you'll see a bunch of warnings and errors on their website. This example pretty clearly shows that Google doesn't care of W3C validation itself. At least not as much to give a strong rank boost to valid websites or penalize erroneous ones. It simply doesn't care. The recommended W3C validation strategy is: perform it to make your site working and accessible with all common browsers and don't bother doing it for the SEO purposes only, if you don't experience any cross-browser issues - it works fine as it is.