different for each element, the rem is constant throughout the document. See also Em dash (—) En (typography) Fullwidth forms List of XML and HTML character entity references Non-breaking space width variations Pixels-per-em (PPEm) – Used in operating systems to describe the allotment of pixels to em height Responsive web design Whitespace characters References External links CSS Length and Units CSS Units and Values The amazing em unit and other best practices An EM calculator/converter The Formula of converting PX to EM Display technology Typography Units of length