In CSS, both em and rem are units of measurement for font sizes, but they differ in their reference point.
em is a relative unit of measurement that is based on the font size of the parent element. For example, if the font size of the parent element is 16 pixels, and you set the font size of a child element to 1.5em, the child element's font size will be 24 pixels (1.5 times the parent's font size).
rem, on the other hand, is also a relative unit of measurement, but it is based on the font size of the root element, which is typically the <html> element. For example, if the font size of the root element is 16 pixels, and you set the font size of an element to 1.5rem, the font size will be 24 pixels (1.5 times the root element's font size).
The main difference between em and rem is that em is relative to the font size of the parent element, whereas rem is relative to the font size of the root element. This means that rem is more consistent across the entire document, while em can be affected by the font size of any ancestor elements.
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