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.

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 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 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 "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.

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 grid-gap property is used to specify the gap or spacing between grid items (rows and columns) in a grid layout. It helps in controlling the spacing and alignment of items within the grid.

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.

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.

SCSS functions can be used to perform mathematical calculations, such as addition, subtraction, and multiplication, before the CSS is compiled. This allows you to generate dynamic and responsive styles based on input values.

The correct CSS function that enables you to perform arithmetic calculations directly within your stylesheets is calc(). This function is particularly useful for dynamically adjusting styles based on calculations, making it a powerful tool for responsive design and layout adjustments.

When using CSS to mask an element, the area defined by the mask image that is transparent will be visible. A mask hides or reveals parts of an element based on the transparency of the mask image. Areas in the mask image that are transparent reveal the underlying content.