Put up a web page in ",different logical sectors" is very useful and reccomended because each team element can working on his logical sector and also we saving a lot of bits, making light the page.
The W3C standard directives for CSS there are since 1996 and now are the standard definitions for CSS, HTML, XHTML and XML documents.
In other words the CSS use, have been needful to separate the matter from the page styles, to favorite the authors of HTML web pages by a clear and separate development system.
In 1993, before that the CSS were used, Netscape Navigator and Internet Explorer, the two browsers that have disputed the "browsers' war" have got some of owners tags, therefor not standardized and not compatble with their competitors.
Here, there are the three main handicaps of this method: