In SASS/SCSS, the @mixin directive allows for the creation of reusable chunks of CSS. Mixins are blocks of styles that can be included and reused in different parts of your stylesheet. They are a powerful way to keep your styles modular and maintainable.

To make an animation run in reverse order, you should use the animation-direction property and set it to alternate. This will cause the animation to play in the reverse direction after each iteration, creating a back-and-forth effect.

In a CSS keyframe animation, the "0%" or "from" keyframe represents the starting point of the animation. It defines the initial state of the animated element. You can set CSS properties and values for this keyframe to specify how the element should look at the beginning of the animation.

When two conflicting styles affect an element, the browser prioritizes the style declared in the last stylesheet loaded. This is known as the "cascading" nature of CSS, where later declarations override earlier ones. Specificity comes into play only if the conflicting styles are in the same stylesheet.

The :first-child pseudo-class in CSS is used to target elements that are the first child of their parent element. It is commonly used to apply styles to the first element in a list or container. This selector is useful for creating specific styles for the first child within a parent.

To ensure that the animation effects persist after the animation completes, you should set the "animation-fill-mode" property to "forwards." This will make the animation retain its final state after completing its execution.

The CSS property used to create animations by gradually changing from one set of CSS styles to another is keyframes. Keyframes are used to define a sequence of style changes over a specified duration using the @keyframes rule. The animation duration can be controlled by the animation-duration property.

In CSS, class selectors have higher specificity than type selectors. Therefore, if a class selector rule is not being applied, but a rule with three type selectors is, it's likely because the class selector rule has a lower specificity.

'vw' stands for Viewport Width in CSS. It is a relative unit that represents a percentage of the width of the viewport, making it useful for creating responsive designs that adapt to different screen sizes.

To keep the header at the top of the viewport when users scroll, you should use the "position: fixed" property. This makes the element's position relative to the browser window, ensuring it stays in place while the page content scrolls beneath it.