The rem unit in CSS is relative to the root element's font size, which makes it a practical choice for creating responsive layouts. Unlike em, which is relative to the font size of its nearest parent, rem is always relative to the font size of the root (html) element.

You define the number of columns in a grid layout by specifying the grid-column-count property. This property allows you to set the exact number of columns you want in your grid, providing fine control over your layout.

To target every third list item in a list, you should use the :nth-child(3n) pseudo-class. This selects elements that are at positions that are multiples of 3 within their parent.

To ensure that text remains visible during web font loading, you can set the "font-display" property in your CSS to "swap." This tells the browser to use a fallback font until the web font is fully loaded and ready to display, preventing a "FOUT" (Flash of Unstyled Text) for web fonts.

The value of the display property that makes an element behave like a block-level element but allows it to sit inline with other content is inline-block. This is particularly useful when you want an element to have block-level properties (like the ability to set width and height) while still flowing inline with adjacent content.

To create a media query for screens wider than 768px, you should use the @media rule, followed by the screen keyword (optional) and and (min-width: 768px). The correct option is @media screen and (min-width: 768px) { /* Styles go here */ }.

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 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.