The calc() function in CSS allows you to perform mathematical operations to calculate values for CSS properties. It's useful for creating responsive layouts and defining dynamic values based on other property values. For example, you can use calc() to set the width of an element as a percentage of the parent container's width minus a fixed value.

To align text to the center in CSS, you can use the text-align property and set it to the value center. This will horizontally center-align the text within its containing element.

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.

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.