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.

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 make an animation run indefinitely in CSS, you should use the animation-duration property and set it to infinite. This property specifies the time it takes for one cycle of the animation, and when set to infinite, it ensures that the animation continues to run indefinitely.

The "font-display" property in CSS controls how web fonts are displayed during loading. It's used to manage the flash of unstyled text (FOUT) or invisible text, making it an essential property for optimizing web font performance. Setting it to "swap" tells the browser to use a system font until the custom font is available, reducing FOUT.

The property that allows you to control the size of tracks (rows and columns) in a grid layout is 'grid-template-columns' for columns and 'grid-template-rows' for rows. These properties let you define the size of the grid tracks using various units like pixels, percentages, or 'fr' units.

The ::after pseudo-element in CSS is used to insert content after the selected element. It is commonly used for adding decorative elements or additional content after an existing element. By using the ::after pseudo-element and specifying the content property, you can dynamically generate and place content after an element without modifying the HTML structure.

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.

'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.

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.

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.

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.

In terms of specificity, ID selectors have the highest priority. They are more specific than class selectors, element selectors, and universal selectors, and styles defined using ID selectors will override styles defined with lower-specificity selectors.