PostCSS is a popular tool used in web development to automatically add vendor prefixes to CSS properties. It is highly configurable and can be integrated into your build process. PostCSS, along with plugins like Autoprefixer, streamlines the process of making your CSS code cross-browser compatible by adding the necessary prefixes.

SASS (Syntactically Awesome Style Sheets) is primarily used in web development for simplifying and organizing CSS. It introduces features like variables, nesting, and mixins, making your CSS more maintainable and efficient.

The CSS clamp() function is used to restrict a value between a specified minimum and maximum value. It allows for responsive design by ensuring that the value falls within this range, helping to maintain a consistent layout across different screen sizes.

To vertically center an inline or inline-block element relative to its parent, you'd set the "vertical-align" property to middle. This property allows you to control the vertical alignment of inline-level elements within their line boxes.

BEM (Block Element Modifier) methodology recommends using an underscore (_) to separate the block from the element in class names. This naming convention enhances code readability and helps identify the hierarchy of elements within a block. For instance, a button element inside a header block might be named header_button.

To optimize scroll performance, you can use the will-change property to hint to the browser that an element will change frequently. This allows the browser to prepare for the changes and allocate resources accordingly, improving scrolling performance. While using transform: translate3d(0,0,0); can also trigger hardware acceleration, it may not be as specific or efficient as will-change. z-index and position: fixed; are unrelated to hinting to the browser about frequent changes in an element.

When an element's position is set to absolute and no ancestors are positioned (with relative, absolute, or fixed positioning), it is positioned relative to the viewport, which is the browser window. This means it's positioned in relation to the visible area of the web page.

To select list items that are the only child within their containing unordered list, you should use the :only-child pseudo-class. This targets elements that are the only child of their parent, ensuring that it's the sole element within the parent container.

Media queries with the orientation feature are used to detect whether the device is in portrait or landscape mode. This is valuable in responsive web design as it allows you to adjust the layout and styling based on the device's orientation, ensuring a better user experience.

The calc() function in CSS allows you to perform mathematical operations to calculate values for CSS properties. It's useful for creating responsive layouts and defining dynamic values based on other property values. For example, you can use calc() to set the width of an element as a percentage of the parent container's width minus a fixed value.