How to resolve the CSS file invalidation method

CSS is correct, the link is not wrong, why the CSS style does not take effect

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd "><! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 strict//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" ><! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 frameset//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-Frameset.dtd" >

Transitional type: Refers to a type of transition that uses this type of browser to ease the parsing of XHTML, allowing the use of tags in HTML4.01, but must conform to the XHTML syntax. This is now a common method, which is the default type when creating a Web page with Dreamweaver. aa25.cn
Strict type: strict type, when used, the browser will be relatively strict, do not allow the use of any representation of the identity and attributes, such as the direct use of bgcolor background color properties in the element.
Frameset Type: The frame page type, which is necessary if the Web page uses a frame structure.
The basic situation is finished, now know why CSS does not take effect, don't look so a word, can not ignore yo!