In responsive web design, the @media rule is used to set breakpoints. It allows you to define specific CSS styles that apply when certain conditions are met, such as screen width or orientation. By using @media queries, you can create responsive layouts that adjust to different screen sizes, making your website look and function well on various devices.

An element with display: none; removes the element from the layout entirely. It doesn't occupy any space, and other elements act as if it's not there. In contrast, visibility: hidden; hides the element but still reserves its space in the layout.

Autoprefixing in CSS development is primarily used to automatically add vendor prefixes to CSS properties. These prefixes are necessary to ensure cross-browser compatibility. When you use CSS properties like "flex" or "grid," different browsers may require different prefixes (e.g., -webkit-, -moz-, -ms-) to render these properties correctly. Autoprefixing tools simplify this process, making your CSS code work consistently across various browsers.

The flex-wrap property in a flex container controls how flex items wrap when they exceed the container's width. It can be set to nowrap to prevent wrapping, wrap to allow wrapping in a single line, or wrap-reverse to wrap in the opposite direction.

The "mobile-first" approach in responsive design involves starting the design process with a focus on mobile devices. This approach emphasizes designing for small screens first and then progressively enhancing the design for larger screens, such as tablets and desktops. It ensures that the design works well on mobile devices and then adapts gracefully to larger screens, providing a better user experience across different devices.

The "line-height" property in CSS specifies the vertical space between lines of text within a block. A larger line-height value increases the vertical spacing, making text more readable, while a smaller value reduces the spacing. It is crucial for controlling text layout and readability.

To make list items display horizontally (side by side) instead of vertically (stacked), you should adjust the CSS property "float: left." This property is used to float elements next to each other.

The mix-blend-mode property in CSS is used to control how elements blend when they overlap. It's used to specify how the colors of elements should be mixed, based on a defined blending mode, to create various visual effects.

SASS/SCSS allows you to use variables to define styles and apply them conditionally. In this scenario, you should use SASS/SCSS variables to define common styles and then create a single class with conditional styles based on these variables to apply the slight variations to multiple elements. This promotes code reusability and maintainability.

The steps value of the animation-timing-function property divides the animation into a series of distinct intervals, creating a frame-by-frame animation effect, similar to a flipbook. You specify the number of steps, like steps(4), to control the number of frames.

To make an animation run indefinitely, you should set the animation-iteration-count property to the value infinite. This ensures that the animation will keep running continuously without a specified end point.

To create a function in SASS that returns a value, you use the @function directive and the @return keyword within the function block. This allows you to define reusable pieces of logic that can compute and return values. Functions are typically used for calculations and value generation in SASS.