Styled Components generate unique class names for styles by using hashing algorithms. This ensures that styles are isolated and don't interfere with other components, a technique often referred to as "CSS-in-JS" or "CSS Modules."

The animation-fill-mode property is used to control the behavior of the animated element before and after the animation execution. It can have values like "none," "forwards," "backwards," and "both." "None" is the default value, which means the element doesn't maintain any styles before or after the animation. "Forwards" retains the styles after the animation ends, and "backwards" maintains the styles before the animation starts. "Both" keeps the styles both before and after the animation.

To apply an animation to an element in CSS, you use the animation-name property to specify the name of the animation defined with the @keyframes rule. This property associates the element with the animation it should use.

To structure and integrate separate SCSS files, you should use the @import directive in the main SCSS file. Import the separate SCSS files for variables, mixins, and base styles at the beginning of the main SCSS file. This allows you to maintain a modular and organized codebase.

When two CSS rules conflict and apply to the same element, the last rule encountered in the stylesheet takes precedence. This is known as the "cascading" nature of CSS.

In SASS or SCSS, the ampersand (&) symbol is used to reference the parent selector within a nested rule. This is useful for creating more specific or complex selectors based on the parent selector's context. For example, you can use &:hover to apply styles when the parent element is hovered.

In Styled Components, you can dynamically change the style of a component based on its props by using the style prop. You can pass in an inline CSS object and conditionally apply styles based on the values of the props. This allows for the creation of versatile and reusable components.

The default value of the 'box-sizing' property in CSS is 'content-box.' In this mode, the width and height of an element do not include padding or borders, only the content is considered. This can sometimes lead to unexpected layout behavior, which is why 'border-box' is often preferred.

Excessive nesting in SASS or SCSS can lead to bloated and inefficient CSS files, which are larger in size. This negatively impacts loading times and page performance. It is advisable to avoid deep nesting to keep the stylesheet lean and maintainable.

A font's loading performance can be improved by "preloading." Preloading involves using the link element in the HTML to fetch and cache font files in advance, reducing the time it takes to load and render text with custom fonts.