We are talking about valid code. Valid code is code that passes validation; a process of checking your documents against a formal standard, like those published by the World Wide Web Consortium (W3C). A webpage that has been checked and passed is considered valid.

From an SEO perspective ensuring that your website pages validate to W3C requirements is important because a consequence of poorly coded pages is poor search engine indexing.

Poor search engine indexing can lead to chunks of your content not being index and therefore not appearing in search engines to be found. Not being findable is of course the kiss of search engine death.

Some common validation errors:

1. No Document Type Definition (DTD) declaration
2. Lowercase DOCTYPE
3. Missing closing ” /”
4. Unescaped ampersand (“&”)
5. Uppercase tags
6. Incorrect nesting of elements
7. Missing required attribute
8. Unquoted attribute values
9. Omitted end tag
10. End tag not open

If a document is missing a closing tag or a Document Type Declaration, modern browsers, especially Internet Explorer, which is very adept at interpreting erroneous code, will usually oblige by displaying the page the way it was intended to be seen. This has spawned a dependency on browser technology to interpret code and display pages correctly. If it looks good, it must be all good. Wrong.

Search engine spiders care that your page does not validate, they are the most basic text browsers and they need a free and easy ride through your code; remember, just because human visitors can read your page it doesn’t mean search engines can.

You need the search engine spiders to visit and have a clear pathway through your page, your visibility online is dependent on the quality of their visits. Coding errors and invalid mark-up can also cause problems with layout and accessibility which means you could also be turning away potential customers from your site who may never to return and that’s only if they can find the site in the first place (see spidering problem…).

In part 2: Steps you can take towards valid code and what you can do to ensure as few errors as possible.