html is markup or syntax (html is based on sgml which is/was used for newsprint). back in html 3.2, html used markup for styling. this lead to a vast array of problems; most notably extremely bloated markup that had no real purpose beyond visual aesthetics. css was developed to remove these problems and to streamline presentation and markup. due the way css was designed, it has been used in a wide array of different markup languages besides html.
in a nutshell, html and css were one in the same, but are completely different from one another. html has the capability to style markup, but that it is not the purpose for which it was intended. css by itself cannot fill the role of markup.