Cascading Style Sheets
HTML code provides page structure for the Browser to interpret into a hierarcy of elements called the Document Object Model or DOM. Cascading style sheets (CSS) provide an organized way to apply margins, coloring, and proportions to that structure. Cascading means styling that is closest to a DOM element has priority over styling either in "head" tag or in a separate file. A separate file styling has the least priority. Also within a file, the lower styles for the same assignment has priority so a selector for an h1 tag lower in the CSS file has the affect cancelling the affect of the same selector above. This also applies to a list of "link" tages in the "head." The sequence of priorities is from top to last in the list. This can cause confusion if you're not aware of this system.
CSS has an unmistakable power to make or break the design of what the user sees and the impact in general of the material presented on a page or in the case of a Landing site, the whole feel of the site.
It would be a mistake to ignore javaScript here because it work in conjunction with the DOM elements and the CSS styling to provide many interactive features. It best to take a course that uses HTML/CSS and JS to better understand what's going on. Once again YouTube is good place to start.