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.

The space between the content of an element and its border is determined by the "padding" property in CSS. Padding provides the spacing between the content and the border of an element, allowing you to control the inner spacing.

To ensure that fonts are loaded from the same origin, you can use the Access-Control-Allow-Origin HTTP header. This header specifies which origins are permitted to access a resource, which is useful for preventing cross-origin resource sharing issues.

To create a fluid typography system with specified limits and smooth scaling based on viewport width, you should use the clamp() function. The correct option is clamp(16px, (3vw + 1vh), 32px), which ensures the font size stays between 16px and 32px while smoothly scaling with viewport width changes.

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.

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.

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 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.

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".