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.

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 "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.

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 mask-composite property in CSS is used to determine how a mask image should be combined with the element it's applied to. It's used to control the blending mode of the mask image with the element, allowing for various effects.

To make a row of items wrap onto the next line when there isn't enough space in a Flexbox layout, you would adjust the flex-direction property to "column" (for vertical stacking) and flex-wrap to "wrap." This allows the items to stack vertically when space is limited.

To pause an animation before it begins or between iterations, you should use the animation-play-state property. By setting it to paused, you can temporarily halt the animation. This is useful for creating interactive animations and controlling when they start or stop.

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.

In CSS, when a variable is not set or its value is invalid, it will return the default value. This default value varies depending on the property. For example, the default value for the font-size property is often 16 pixels.

The font-family property in CSS is used to specify the font family for an element. This property allows you to define the typeface or font you want to use for text within that element, which can greatly affect the visual style of your website.