Most standards aware web professionals can be found here. These people tend to think about structure and semantics first and presentation later. Strict doctypes are generally used at this level to encourage the separation of semantic and presentational markup. Whether the markup language is HTML 4.01 or XHTML 1.0 is less important to many. In fact, some people at Level 5 are questioning the use of XHTML. Level 5 people tend to get into endless discussions about tiny markup details. They can waste hours thinking about class names or reorganising their HTML and CSS files to make their code look pretty.

If you have reached this level I think you’re doing extremely well. For most people there is no compelling reason to move beyond this Level 5.

Read about all the 6 level here