Many webmasters overlook a very important aspect of web site promotion: the validity of the HTML code.
What is valid HTML code?
Most web pages are written in HTML. As for every language, HTML has its own grammar, vocabulary and syntax, and every document written in HTML is supposed to follow these rules.
Like any language, HTML is constantly changing. As HTML has become a relative complex language, it's very easy to make mistakes. HTML code that is not following the official rules is called invalid HTML code.
Why is valid HTML code important?
Search engines have to parse the HTML code of your web site to find the relevant content. If your HTML code contains errors, search engines might not be able to find everything on the page.
Search engine crawler programs obey the HTML standard. They only can index your web site if it is compliant to the HTML standard. If there's a mistake in your web page code, they might stop crawling your web site and they might lose what they've collected so far because of the error.
Although most major search engines can deal with minor errors in HTML code, a single missing bracket in your HTML code can be the reason if your web page cannot be found in search engines.
If you don't close some tags properly, or if some important tags are missing, search engines might ignore the complete content of that page.
How can you check the validity of your HTML code?
Connect to an official HTML validator that will check the code of your web pages.
Although not all HTML errors will cause problems for your search engine rankings, some of them can keep web spiders from indexing your web pages.
Valid HTML code makes it easier for search engine spiders to index your site so you should make sure that at least the biggest mistakes in your HTML code are corrected.