What unit of measurement in CSS is used to define responsive font sizes?

Using ems as a unit of measurement in CSS for defining responsive font sizes is particularly effective because it is a relative measurement based on the font size of the element's parent. When you use ems, the size of the font can scale based on the user's settings or the surrounding context, making your design more flexible and adaptable across different devices and screen sizes.

For example, if the parent element’s font size is set to 16 pixels, setting a font size of 2 ems would result in a font size of 32 pixels. As a result, when the parent element’s font size scales, any child elements using ems will also adjust accordingly. This adaptability allows for better accessibility and user experience, as users can change the base font size to meet their visual preferences without breaking the layout.

In contrast, other measurement units like pixels are fixed and do not accommodate changes in user settings or screen sizes as effectively, making them less suitable for responsive design. Points and inches are mainly used for print styling rather than on-screen contexts, reinforcing that ems are the preferred choice for responsive typography.