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.

The attr() function in CSS is employed to retrieve the value of an HTML attribute. This is particularly useful when you want to style elements based on the values of their attributes, such as custom data attributes or standard HTML attributes like "href" or "src".

To position an item in the third row and second column of a grid, you use the grid-row and grid-column properties. For example, you can set grid-row: 3 and grid-column: 2 to achieve this precise placement.

A "FOUT" (Flash of Unstyled Text) occurs when web fonts take a bit of time to load, and during this time, the browser displays text in the default font before switching to the web font. This can be mitigated by using techniques like font preloading or font-display property in CSS.

To prevent a block of text from wrapping to the next line, you would set the white-space property to nowrap. This property prevents text from breaking at spaces or hyphens, keeping it on a single line.

To make the width of the button remain constant, regardless of its padding and border, you would use width: min-content. This value ensures that the button's width is only as wide as its content, ignoring padding and border.