Some perceptions

DOCTYPE(document type), it's quite simply an element used to declare what language (and its level) a document uses,
1.Transitional: !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd>
2.Strict: !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
3.Frameset: DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">　
The relationship between them like modle and clothes. Modle is datas, clothes is form. They are separated, then you can change the clothes for your modle as you wish. However, in the html4(transitional), they are mixed together. It gets more difficult to change your model's clothes.