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.

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.

To include a custom font in your webpage using CSS, you use the font-family property and specify the font's name. You need to first import the font into your CSS file or link to an external font file, and then you can reference it using the font-family property in your CSS rules. This allows you to apply the custom font to specific text elements on your webpage.