To optimize scroll performance, you can use the will-change property to hint to the browser that an element will change frequently. This allows the browser to prepare for the changes and allocate resources accordingly, improving scrolling performance. While using transform: translate3d(0,0,0); can also trigger hardware acceleration, it may not be as specific or efficient as will-change. z-index and position: fixed; are unrelated to hinting to the browser about frequent changes in an element.

BEM (Block Element Modifier) methodology recommends using an underscore (_) to separate the block from the element in class names. This naming convention enhances code readability and helps identify the hierarchy of elements within a block. For instance, a button element inside a header block might be named header_button.

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.

To ensure compatibility with modern CSS features, you can use a PostCSS plugin like "autoprefixer" that automatically generates fallback code for unsupported features, allowing you to write modern CSS with confidence. This minimizes the need for manual fallbacks.

To select every even item in a list, you should use the :nth-child(even) selector. This will apply the styling to the elements with even indices in the list, which will give them a gray background in this scenario.