Consider CSS Zen Garden, “a demonstration of what can be accomplished visually through CSS-based design”.
View several of the various designs. Notice that the text content remains the same, despite extreme differences in visual style.
View the source code for several of the designs. Notice that the HTML is nearly identical for all CSS Zen Garden Designs.
The sole difference is a single line of code: the import statement (which loads an external style sheet) is different for each design, because each design gets its own CSS. The external style sheets vary, the HTML remains the same.
Only the CSS changes. The HTML remains exactly the same, other than the import statement.
Images also change, but only images referenced from CSS (never the img tag in HTML).