To provide a fallback value for a CSS custom property, you can use the var() function. The first argument of var() is the custom property, and the second argument is the fallback value. If the custom property is not defined, the browser will use the fallback value. For example, --custom-property: var(--fallback, fallback-value); sets --custom-property to the value of --fallback if it exists, or "fallback-value" if --fallback is not defined.

When 'box-sizing' is set to 'content-box,' the total width of an element includes padding and border. The content width does not include these, but the total width does.

The technique where a low-quality or system font is displayed initially and then replaced with a higher-quality custom font when it's loaded is known as "Font FOUT" or "Flash of Unstyled Text." This approach improves perceived performance and ensures a smoother transition between fonts.

The animation-fill-mode: forwards; property-value combination holds the final keyframe styles after the animation finishes. This means that after the animation completes, the element retains the styles from the final keyframe. This can be useful for maintaining a particular state after the animation finishes.

When an element has a 'position: sticky;' property and scrolls past its container, it sticks to the container until it reaches a specified offset, after which it scrolls along with the container. This behavior is often used for navigation bars.

The property commonly used to control the speed curve of a transition effect in CSS is transition-timing-function. This property allows you to specify how the intermediate property values are calculated over the duration of the transition, giving you control over the timing and pace of the transition.

To create a smooth heartbeat animation that makes an icon grow and shrink, you should use the scale transformation with CSS animations. This allows you to scale an element up and down smoothly, ensuring it runs seamlessly in both directions.

The CSS pseudo-class :not() is used to select elements that do not match a specific criteria. It is used to exclude elements that would typically be selected by a CSS selector. For example, :not(.classname) selects all elements that do not have the specified class.

The CSS rule "ul > li" specifically targets only the direct child li elements of a ul element. It will not target li elements that are nested further within the hierarchy of the HTML structure.

The CSS rule "ul + ul" is a sibling combinator that targets an unordered list that immediately follows another unordered list in the HTML structure. This is used to style a specific occurrence of an unordered list in relation to another.