You can't... HTML and CSS are two completely different things.
HTML builds the structure of a webpage.
CSS controls the style or appearance of the webpage.
The two are not interchangeable or convertible to one another.
Furthermore, what you might be referring to is the way HTML controls appearance. For example, before CSS was widely used, HTML used the <font> tag to control color, size, font typefaces, etc. Now, CSS can be used by applying styles that gives text (or anything else) the same and more properties as HTML.
