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.

Autoprefixing in CSS development is primarily used to automatically add vendor prefixes to CSS properties. These prefixes are necessary to ensure cross-browser compatibility. When you use CSS properties like "flex" or "grid," different browsers may require different prefixes (e.g., -webkit-, -moz-, -ms-) to render these properties correctly. Autoprefixing tools simplify this process, making your CSS code work consistently across various browsers.

An element with display: none; removes the element from the layout entirely. It doesn't occupy any space, and other elements act as if it's not there. In contrast, visibility: hidden; hides the element but still reserves its space in the layout.

In responsive web design, the @media rule is used to set breakpoints. It allows you to define specific CSS styles that apply when certain conditions are met, such as screen width or orientation. By using @media queries, you can create responsive layouts that adjust to different screen sizes, making your website look and function well on various devices.

The primary purpose of the grid-template-areas property is to specify the layout of the grid by assigning a name to grid areas. This allows for a visual representation of the grid structure and helps in creating complex and responsive layouts with CSS Grid.

The device-width feature is commonly used in media queries to detect the width of the viewport. It specifically targets the width of the output device, which is important for creating responsive designs that adapt to different screen sizes. By using device-width in media queries, you can apply different styles and layouts based on the device's screen width.

The primary purpose of the filter property in CSS is to apply visual effects to elements. This property allows you to add various effects like blurring, brightness adjustment, and grayscale to an element. It's commonly used for image effects and enhancing the visual appearance of web content.

The "line-height" property in CSS specifies the vertical space between lines of text within a block. A larger line-height value increases the vertical spacing, making text more readable, while a smaller value reduces the spacing. It is crucial for controlling text layout and readability.