That is not correct. The em unit has always behaved with respect to the inherited font size. If your body has font-size: 16px; then a child with font-size: 1.5em; would appear the same as using 24px for the value. This can recur with nesting, so that 24 could become 36 with another layer added. The rem (root em) unit solves this “problem” as it’s always relative to the document root (usually the font-size you’ve set to html/body)

em was always compounding, but this website used to work in IE because it was generated by Frontpage. Microsoft made tools generating non valid HTML so they would break in Netscape.