In CSS, font-size can be specified in various units, including em, ex, px, and cm. However, 'cm' is not a common unit for font-size. It's typically used for specifying dimensions of other elements, like margins or padding.

The ::first-letter pseudo-element in CSS is used to target the first letter of a block-level element, such as a paragraph or a heading. It's commonly used for styling the initial letter of a text block, for decorative or typographic purposes.

To make an animation play forwards first and then reverse on alternate cycles, you would use the animation-direction property with the value alternate-reverse. This value causes the animation to run normally on odd iterations (forwards) and in reverse on even iterations.

To create a button hover effect where the button scales up gradually, you would primarily use the combination of CSS properties: transform: scale(1.2); for scaling the button and transition: transform 0.3s ease-in-out; to specify the transition duration, easing function, and the property to be transitioned (in this case, "transform"). This combination smoothly scales up the button on hover.

To vertically center a single item inside a container using Flexbox, you would set the align-items property to "center" to vertically align the item within the container, and you would set justify-content to "center" to horizontally center the item within the container.

In Styled Components, a popular library for styling React components, styles are written inside JavaScript code using tagged template literals (usually within JSX files). This approach allows developers to encapsulate styles within the components themselves.

Media queries in CSS allow you to conditionally apply styles based on the width (or dimensions) of the device or browser window. By specifying different rules for various screen sizes, you can create responsive designs that adapt to different viewing environments.

PostCSS is a versatile tool that allows developers to create custom transformations or plugins for their projects. These transformations can be used to modify or extend the functionality of CSS, making it a powerful and flexible tool for developers.

The @font-face rule in CSS allows web designers to use fonts that are not installed on the user's machine. This is useful for ensuring consistent typography across different devices.

The CSS unit 'vh' represents a percentage of the viewport's height. This means that 1vh is equal to 1% of the height of the viewport in which the web page is displayed. It's a relative unit often used for responsive design.

The animation-timing-function property is used to control the speed curve of an animation. It allows you to specify how the animation progresses through time, giving you control over the easing and acceleration of the animation.

In CSS transitions, the transition-timing-function property controls the timing function for the transition. The ease-in-out option creates a smooth transition that starts slowly, accelerates in the middle, and slows down at the end. This is a common choice for animations that feel natural and pleasing to the eye.