CSS Classes-
React allows class names to be specified for a component, like class names are specified for a DOM element in HTML.
When developers first start using React after developing traditional web applications, they often use CSS classes for styling because they are already familiar with the approach.
Inline CSS-
Styling React elements using inline CSS allows styles to be completely scoped to an element using a well-understood, standard approach. However, there are certain styling features that are not available with inline styles. For example, the styling of :hover pseudo-classes.
Pre-processors Such as Sass, Stylus, and Less-
Pre-processors are often used on React projects. This is because, like CSS, they are well understood by developers and are often already in use if React is being integrated into a legacy application.