PostCSS is a popular tool used in web development to automatically add vendor prefixes to CSS properties. It is highly configurable and can be integrated into your build process. PostCSS, along with plugins like Autoprefixer, streamlines the process of making your CSS code cross-browser compatible by adding the necessary prefixes.

To vertically center an inline or inline-block element relative to its parent, you'd set the "vertical-align" property to middle. This property allows you to control the vertical alignment of inline-level elements within their line boxes.

SASS (Syntactically Awesome Style Sheets) is primarily used in web development for simplifying and organizing CSS. It introduces features like variables, nesting, and mixins, making your CSS more maintainable and efficient.

The CSS clamp() function is used to restrict a value between a specified minimum and maximum value. It allows for responsive design by ensuring that the value falls within this range, helping to maintain a consistent layout across different screen sizes.

The "animation-direction" property in CSS allows you to control the direction of an animation. Setting it to "reverse" will make the animation play in the reverse direction, essentially rewinding the animation.

To target both an ordered list (ol) and its list items (li) in a single CSS selector, you should use "ol li" without any comma or additional characters. This selector will apply styles to all li elements within an ol element.

The value of animation-fill-mode that ensures the animation's styles are applied before the animation begins is backwards. When you set animation-fill-mode to backwards, the element retains the styles of the first keyframe until the animation begins. This can be useful to set initial styles before the animation starts.

To create a sidebar that sticks to the top after scrolling past its initial position but stops sticking when it reaches the footer, you should apply the "position: sticky" property. It maintains the element's natural position until a specified scrolling threshold is reached, and then it behaves as if it were fixed.

The "forwards" value for the "animation-fill-mode" property makes the animation continue in its final state after completion. It ensures that the final keyframe of the animation is retained as the end state even after the animation completes.

When certain properties like opacity and transform are animated, they are considered more performant because they typically don't trigger relayouts or "reflows" in most browsers. Relayouts are costly in terms of performance and can significantly slow down animations.