Final answer:
When using percentages for setting sizes in CSS, width is usually relative to the width of the containing element, while height is relative to the height of the containing element.
Step-by-step explanation:
When percentages are used to size height and width in CSS, the dimensions are set relative to different elements based on the property being used. For width, it is usually relative to:
- The width of the containing element (if width is the property being set in percentages).
- The browser window or viewport width (if max-width or min-width properties are used).
In the case of height, the percentage is usually relative to the height of the containing element. It's important to note that if the containing element's size isn't defined, the height percentage may not work as expected, as it may have no point of reference.