The @font-face rule in CSS allows web designers to use fonts that are not installed on the user's machine. This is useful for ensuring consistent typography across different devices.

PostCSS is a versatile tool that allows developers to create custom transformations or plugins for their projects. These transformations can be used to modify or extend the functionality of CSS, making it a powerful and flexible tool for developers.

The CSS property that allows you to set a graphical element to be used as a mask against another element is mask-image. This property lets you specify an image or SVG as a mask, which defines the transparency of an element based on the mask's alpha channel. It's often used for creating complex visual effects and overlays.

Styled-components is a popular CSS-in-JS library for React. It allows you to define component styles directly in your JavaScript code and can easily style components based on props, making it ideal for dynamic styling based on prop values.

The default value for animation-timing-function is ease. This timing function provides a smooth and gradual acceleration at the beginning and a deceleration at the end of the animation, creating a more natural and visually appealing effect.

To achieve a cursive style for text in CSS, you should use the font-family property with the value set to 'cursive'. This will apply a cursive font family to the selected text, which is suitable for vintage-themed designs.

The "line-height" property in CSS can be set using various units. While "px" and "em" are commonly used, it can also be set using a percentage ("%"). This percentage value is relative to the font size of the element, making it a versatile option for controlling line height.

To pass values when including a mixin in SASS/SCSS, you use the @include directive followed by the mixin name. The correct syntax is @include myMixin();. This allows you to apply the styles defined in the mixin and pass any necessary values as arguments inside the parentheses.

The default value of the flex-direction property in Flexbox is row. This means that flex items are laid out in a row, horizontally. If you want them to stack vertically, you need to set flex-direction to column.

To double the value of a CSS variable named --gap and use it as a margin, you should use the calc() function within the margin property. The correct option is margin: calc(var(--gap) * 2);, which properly references and doubles the variable value.