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.

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.

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.

The ::before pseudo-element in CSS is used to generate content before the content of an element. It creates an element that appears as the first child of the selected element and allows you to insert generated content (such as text or images) before the actual content of the element.

To make an element inherit a property's value from its parent element, you would use the 'inherit' value. This ensures that the child element inherits the property value from its parent.

Styled Components generate unique class names for styles by using hashing algorithms. This ensures that styles are isolated and don't interfere with other components, a technique often referred to as "CSS-in-JS" or "CSS Modules."

The CSS property "line-height" primarily affects the spacing between lines of text within an element. It determines the vertical distance between the baselines of two consecutive lines of text. Adjusting the line-height can impact the readability and aesthetics of text on a webpage.