Answer
px, em, rem, and percentages use different reference sizes. • A CSS pixel is an absolute CSS unit, not necessarily one device pixel. • em uses the relevant element font size; rem uses the root element font size. • Percentages use the reference defined by each property.
💡 Simple Example
.title { font-size: 2rem; margin-block: .5em; width: 80%; }
⚡ Quick Revision
px is absolute; em, rem, and percentages are context-relative.