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 include a mixin in your SCSS code, you use the @include directive followed by the mixin name inside parentheses. For example, @include myMixin(); will include the styles defined in the myMixin mixin at that point in your stylesheet. You can also pass arguments to the mixin if it accepts them.

To make the text of an element bold in CSS, you should use the font-weight property and set it to the value bold. This increases the thickness of the characters, making them appear bold.

To target an element that is the only child of its parent, you can use the :only-child pseudo-class. This selector is used when you want to apply styles or behavior to an element that is the only child within its parent container. For instance, you might want to style a single button in a list of buttons.

Opting for the "woff2" font format over others can improve web performance because WOFF2 files are generally smaller in size. Smaller files mean faster downloads and quicker web page rendering, resulting in a better user experience.

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.

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.

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.

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.

Object-Oriented CSS (OOCSS) emphasizes separating the container and content in your styles and promoting the reuse of styles (i.e., CSS rules). This approach helps keep your CSS modular, making it easier to maintain and reducing redundancy in your code.

To ensure that hyperlinks have no underlines by default but are underlined when hovered over, you should use the CSS rule a { text-decoration: none; } to remove the underlines from all anchor elements and then use a:hover { text-decoration: underline; } to specify that underlines should appear when the link is hovered.

The property in CSS that allows you to apply a delay before an animation starts is the animation-delay. This property is used to create a pause before the animation begins, which is especially useful for sequencing animations.