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.

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.

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.

The em unit in CSS is relative to the font-size of the parent element. It's a relative unit, making it valuable for creating scalable and responsive designs. When you set an element's font-size in ems, it's relative to its parent's font-size.

To ensure that a CSS transition applies only when a specific property changes, you can use the transition-property property and set it to "all." This means the transition will be applied to all properties that change. If you want to target specific properties, you can list them individually in the transition-property property. Using "all" is a convenient way to apply transitions to all changing properties.