1. Naming Convention: The key difference between BEM and SUIT CSS lies in their naming conventions. BEM (Block Element Modifier) uses a unique naming approach where each component is named based on its functionality or purpose. It follows a hierarchical structure by appending prefixes or suffixes to the class names to indicate the relationship between elements. SUIT CSS, on the other hand, follows a more modular approach by using a flat naming convention. It avoids any hierarchical structure and focuses on using single class selectors with concatenated names that describe the component's purpose.
2. Selectors: In terms of selectors, BEM uses complex selectors to target specific elements within a component. It relies on descendant selectors, child selectors, or modifier classes to style different parts of the component. SUIT CSS, however, promotes a more direct approach by using only class selectors without combinators or descendant selectors. It aims to keep the styles encapsulated and maintainable by avoiding nested or complex selectors.
3. Component Scope: BEM emphasizes the scope of a component by encapsulating all the styles within the component's class. This allows the styles to be self-contained and reusable across different projects. SUIT CSS takes a different approach by focusing on composability rather than strict encapsulation. It encourages the use of utility classes that can be composed together to create different component variations. This promotes code reuse and reduces the need for rewriting similar styles for different components.
4. Customization: BEM provides a higher level of customization as it allows for easy modification of individual elements within a component. By using modifiers, developers can easily change the appearance or behavior of specific elements without affecting other parts of the component. SUIT CSS, on the other hand, promotes a more holistic approach to customization. It encourages the use of theme variables and utility classes that can be applied globally to achieve consistent styling across components.
5. Browser Support: BEM is widely supported by all modern browsers since it relies on standard CSS selectors and class names. SUIT CSS, being a more recent approach, may encounter some challenges with older browsers that do not support the latest CSS features such as flexbox or custom properties. However, with the help of tools like PostCSS and CSS preprocessors, these compatibility issues can be addressed.
6. Learning Curve: BEM has a steeper learning curve due to its complex naming conventions and hierarchical structure. It requires understanding the various naming conventions, the relationship between elements, and the appropriate usage of modifiers. SUIT CSS, on the other hand, has a more straightforward learning curve as it follows a flat naming convention and promotes a modular approach. It focuses on creating reusable utility classes that can be easily composed together.

In summary, BEM and SUIT CSS differ in their naming conventions, selectors, component scope, customization approach, browser support, and learning curve. BEM uses a hierarchical structure and complex selectors, emphasizing scope and customization, while SUIT CSS follows a flat naming convention, promotes composability, and focuses on utility classes.