Turn a pixel size into em (or rem) against a root font size, usually 16 px.
| em | – |
Use this for CSS when a design is in pixels and the stylesheet wants relative units. Type the pixel size and the base font size.
em = px ÷ base. With a 16 px root, 24 px is 1.5 em. This is a simple ratio, not a full cascade of nested ems. The same number is rem if the base is the root font size.
Pixels: The size in pixels you want to express as em.
Base font (px): The root (or parent) font size in pixels, often 16.
em: The relative size, px ÷ base.
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