The em unit in CSS is relative to the font-size of the parent element. It's a relative unit, making it valuable for creating scalable and responsive designs. When you set an element's font-size in ems, it's relative to its parent's font-size.

To ensure that a CSS transition applies only when a specific property changes, you can use the transition-property property and set it to "all." This means the transition will be applied to all properties that change. If you want to target specific properties, you can list them individually in the transition-property property. Using "all" is a convenient way to apply transitions to all changing properties.

The animation-fill-mode property is used to control the behavior of the animated element before and after the animation execution. It can have values like "none," "forwards," "backwards," and "both." "None" is the default value, which means the element doesn't maintain any styles before or after the animation. "Forwards" retains the styles after the animation ends, and "backwards" maintains the styles before the animation starts. "Both" keeps the styles both before and after the animation.

To apply an animation to an element in CSS, you use the animation-name property to specify the name of the animation defined with the @keyframes rule. This property associates the element with the animation it should use.

To structure and integrate separate SCSS files, you should use the @import directive in the main SCSS file. Import the separate SCSS files for variables, mixins, and base styles at the beginning of the main SCSS file. This allows you to maintain a modular and organized codebase.

When two CSS rules conflict and apply to the same element, the last rule encountered in the stylesheet takes precedence. This is known as the "cascading" nature of CSS.

In SASS or SCSS, the ampersand (&) symbol is used to reference the parent selector within a nested rule. This is useful for creating more specific or complex selectors based on the parent selector's context. For example, you can use &:hover to apply styles when the parent element is hovered.

The ">" combinator is used to target a direct child element, ensuring that styles are applied only to the immediate child and not to nested descendants.

The property in CSS that allows you to apply a delay before an animation starts is the animation-delay. This property is used to create a pause before the animation begins, which is especially useful for sequencing animations.

To ensure that hyperlinks have no underlines by default but are underlined when hovered over, you should use the CSS rule a { text-decoration: none; } to remove the underlines from all anchor elements and then use a:hover { text-decoration: underline; } to specify that underlines should appear when the link is hovered.