em – a powerful and easy-to-use font sizes for the site. Whatever the font size of 12px, 16 or 60, the em is always exactly equal to this size. em – Specific unit of measurement in typography. Where was chosen this name? here's some history: em were initially equal to the size metal block in the typewriters that size was equal to the width of a capital 'M' computers use the so-called 'kerning' to highlight the horizontal space for each letter, making the letters equidistant. Initially, the metal block was cut or "kerned", to make a horizontal addition around each letter the same. For example, in CSS, em – in fact, the vertical dimension.
One em is equal to the vertical post, required for any letter in the font, regardless of horizontal space that it occupies. So: If the font size of 16px, then 1em = 16px. All popular browsers default font size 16px. Therefore, if your browser to default, 1em = 16px. So, before the CSS selector is described, the default font size in brauezer 16px. inherit it if not described more in the CSS. Therefore, 1em = 16px, 0.5em = 8px, 10em = 160px, and so on. We can now determine the size of any element that we need to by em!