To style only the first list item of an unordered list in CSS, you should use the ul li:first-child selector. This targets the first li element within a ul element. It's important to note that :first-child targets the first child of its parent, not the first of its type.

The CSS property used to clip an element to a basic shape like a circle or ellipse is clip-path. This property allows you to define a clipping path that restricts the visible area of an element to the specified shape. It's commonly used for creating non-rectangular shapes. border-radius is used to round the corners of elements, not for clipping.

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.

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 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.

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.

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."

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.

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.

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.