All markup and output from Mura CMS is based on Web Standards, and is logically structured and semantically correct.
You can learn more about Web Standards at http://en.wikipedia.org/wiki/Web_standards
It is important to keep in mind that there is nothing "special" about markup from Mura CMS. If you are converting a site to Mura that follows Web Standards, your conversion should go very smoothly. That being said, converting a site that doesn't follow Web Standards is also possible, but you may have to either customize Mura to follow your own standards, or update your markup to follow Web Standards.