In web development, the "font-display" property is used to control how fonts are displayed. The value swap tells the browser to render text with system fonts until the custom font is loaded, making the text remain invisible until the custom font is ready.

In CSS, the 'width' property is used to set the width of the content area of an element. It controls the width of the element's content, excluding padding, border, and margin.

To use a custom property in your styles in CSS, you reference it with the @ symbol. Custom properties, also known as CSS variables, are defined using the :root pseudo-class, and their values can be accessed and used in your styles by prefixing the custom property name with the @ symbol.

The @keyframes rule in CSS is used to specify the keyframes for CSS animations. Keyframes describe how an animation progresses from the starting state to the ending state. They define the intermediate steps or percentage-based changes in the animation's properties, allowing for smooth and controlled animations.

For setting the "line-height" property to ensure consistent spacing, the recommended unit is typically "em." Using "em" units allows the line height to be relative to the font size, which ensures that the spacing remains consistent even if the text size changes. This is especially useful for creating responsive and accessible web designs.

To create a theme switcher in SASS/SCSS, you can define different color variables and use SASS/SCSS functions to return the appropriate color values based on the selected theme. This allows you to switch themes easily by changing a single variable and ensures consistency across your project.

The default value of the animation-direction property in CSS is normal. When animation-direction is set to normal, the animation plays forward, and then it starts from the beginning when it reaches the end, creating a continuous loop.

The clear property in CSS is used to clear the effects of floating elements. It ensures that no element should touch the left or right-hand sides of a floating element and that it should appear below any preceding floating elements.

To use a custom property (also known as a CSS variable), you should use the syntax --property-name. Custom properties allow you to define reusable values in your CSS and use them throughout your stylesheet. They are preceded by two hyphens and are referenced with the var() function, like var(--property-name).

The process by which styles are applied to an element based on their source and specificity is known as the Cascade. Cascade refers to the process of determining which styles should be applied when there are conflicting style rules from different sources like user styles, author styles, and browser defaults.