Which of the following is NOT a CSS property?

The reason that "dimensions" is not a CSS property is that it does not exist as a standalone property in CSS. In CSS, properties that relate to the size of elements, such as width and height, are distinctly defined. Instead of using a property called "dimensions," web developers specify the individual width and height of elements to control their size.

Properties like "color," "background," and "font-family" are all valid CSS properties that stylistically affect the appearance of elements. "Color" controls the text color, "background" modifies the background of elements, and "font-family" sets the font for text within an element. Each of these properties has specific, defined purposes that are widely used in CSS styling.

Thus, knowing the correct terminology and recognizing that "dimensions" doesn't fit into the established CSS property categories is crucial for effective web development and understanding how to apply styles appropriately.