Mixins in SCSS are applied to your styles using the @include directive. This allows you to include the styles defined in the mixin within your CSS rules.

When animated, changing the color property of an element typically causes a "repaint" (updating the pixels on the screen) but not a "reflow" (changing the layout of elements on the page). This is because changing the color doesn't affect the size or position of the element, but it does require repainting.

Setting an element's property to 'initial' means it will use the default value for that property as defined by the CSS specification. This effectively resets the property to its default state.

In CSS, the 'margin' property is used to determine the space outside an element. It creates spacing around the element, separating it from other elements in the layout.

In the SMACSS (Scalable and Modular Architecture for CSS) methodology, states like "is-active" or "is-hidden" are typically handled by applying them as classes directly in the HTML markup. This allows for clear separation of styling and behavior, making the code more maintainable and modular.

PostCSS is a tool used to process CSS and extend it with new features. It allows developers to use future CSS syntax by transforming it into standard CSS. In this context, the answer is "CSS."

For ensuring compatibility with the last two versions of major browsers, Autoprefixer is the ideal tool. It automatically adds vendor prefixes to your CSS code, ensuring that your styles work in various browsers. This is essential for maintaining cross-browser compatibility.

The em unit is relative to the font-size of the element itself, while the rem unit is relative to the font-size of the root element (html). This means that em units can compound if used within nested elements, whereas rem units remain consistent throughout. Understanding this difference is crucial for responsive design.

To style list items that are not the first child, you can use the pseudo-class ":not(:first-child)." This allows you to target and style all list items except the first one in a list.

The CSS property used to vertically align inline elements or inline-block elements is vertical-align. It allows you to control how the element aligns in relation to the surrounding content or other inline elements. It's commonly used for aligning elements such as images and text within a line of text.

The property used to control how inherited values and set values are used by an element and its child elements is the inherited property. It allows for the explicit declaration of whether a property should be inherited by child elements. This is particularly useful for managing the cascading nature of CSS.

To underline text in CSS, you should use the text-decoration property and set it to "underline." This CSS rule adds a line beneath the text.