Can anyone nicely explain what "em" is (a sizing unit of CSS)? -

Can someone really explain that em Is the CSS as the unit?

And when we use the em shape as a unit for a website, why use% for the body? Why not use em for body?

The best way to find

here you can see that in question Element is defined as the font-size, i.e. it is related to the height of the font element is not a font-size measurement for a particular letter. The actual height of individual characters may be more or less than the font-size, although generally they will decrease:

In digital type, the height of specific letters is related to them in an orientation Typeface is determined by the designer. However, as a very strict guideline, the "average" font can have a cap height of 70%, and 48% of them may have an X-height.

Note in another CSS standard:

The only exception to this rule is 'font-size' property where 'em' and 'ex' values View parent element size element

This exception is understandable, otherwise you will get a recursive definition for font size.

One is commonly used, but wrong, the definition is that it is the width in the letter 'M' in the question, it was defined in such a manner in typography, but these are not true these days. , And is never true for CSS. The 'M' is actually less than 1m wide (it depends on the font of the course).