The default value of the animation-direction property in CSS is normal. When animation-direction is set to normal, the animation plays forward, and then it starts from the beginning when it reaches the end, creating a continuous loop.

Media queries in CSS allow you to conditionally apply styles based on the width (or dimensions) of the device or browser window. By specifying different rules for various screen sizes, you can create responsive designs that adapt to different viewing environments.

In Styled Components, a popular library for styling React components, styles are written inside JavaScript code using tagged template literals (usually within JSX files). This approach allows developers to encapsulate styles within the components themselves.

To vertically center a single item inside a container using Flexbox, you would set the align-items property to "center" to vertically align the item within the container, and you would set justify-content to "center" to horizontally center the item within the container.

To create a button hover effect where the button scales up gradually, you would primarily use the combination of CSS properties: transform: scale(1.2); for scaling the button and transition: transform 0.3s ease-in-out; to specify the transition duration, easing function, and the property to be transitioned (in this case, "transform"). This combination smoothly scales up the button on hover.

To make an animation play forwards first and then reverse on alternate cycles, you would use the animation-direction property with the value alternate-reverse. This value causes the animation to run normally on odd iterations (forwards) and in reverse on even iterations.

The ::first-letter pseudo-element in CSS is used to target the first letter of a block-level element, such as a paragraph or a heading. It's commonly used for styling the initial letter of a text block, for decorative or typographic purposes.

In CSS, font-size can be specified in various units, including em, ex, px, and cm. However, 'cm' is not a common unit for font-size. It's typically used for specifying dimensions of other elements, like margins or padding.

SCSS (Sassy CSS) files have the ".scss" extension. This extension is used to indicate that the file contains SCSS syntax, which is a more CSS-like syntax with additional features provided by SASS.

The pseudo-class :nth-of-type(2) in CSS targets the second element of its type. It allows you to select and style the second occurrence of an element type within its parent container. This is useful for creating specific styles for the second element, such as the second paragraph or the second list item.