To move an element on the Z-axis without triggering layout or paint operations, you can use the transform property with the translateZ(0) function. This is often used to create a new stacking context for the element, which can improve rendering performance. It doesn't affect the layout or paint and is ideal for off-screen animations or optimizing 3D transforms.

In SASS, a CSS preprocessor, "nested rules" refer to the practice of defining CSS rules within other rules using indentation. This results in more organized and efficient CSS code, making it easier to understand and maintain. Nested rules help you avoid repetition and improve code structure.

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.

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.

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 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.