CSS provides several units of measurement for specifying font sizes, including pixels (px), ems (em), and root ems (rem). Each of these units has its own advantages and disadvantages when it comes to font sizing.
Pixels
Pixels (px) is an absolute unit of measurement, which means that the font size will be a fixed number of pixels, regardless of the user's browser settings or the device they are using. This can be useful for ensuring that text is always a specific size, but it can also make it difficult to create a responsive design that adapts to different screen sizes.
Ems
Ems (em) is a relative unit of measurement that is based on the font size of the parent element. One em is equal to the font size of the parent element. If the parent element has a font size of 16px, then 1em is equal to 16px. The advantage of using ems is that they allow for easy scaling of text based on the parent element's font size. This is useful for creating responsive designs that adapt to different screen sizes.
Rem
Rem (root em) is also a relative unit of measurement, but it's based on the font size of the root element (usually the <html> element) instead of the parent element. This means that all font sizes specified in rem units will be relative to the root element's font size. This provides a consistent and predictable way to size text across different elements in the page, and also allows for easy scaling of text based on the user's browser settings or the device they are using.
In summary, when it comes to font sizing in CSS, px is an absolute unit of measurement which is fixed, em is relative to the parent element and rem is relative to the root element. Px is useful for ensuring that text is always a specific size, but can make it difficult to create a responsive design, em allows for easy scaling of text based on the parent element's font size and rem provides a consistent and predictable way to size text across different elements in the page while also allows for easy scaling of text based on the user's browser settings or the device they are using.