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Sizing Elements


The second relative unit of measurement in CSS is the rem, coded as rem.

Rem stands for root em. It acts similar to em, but instead of checking parent elements to size font, it checks the root element. The root element is the <html> tag.

Most browsers set the font size of <html> to 16 pixels, so by default rem measurements will be compared to that value. To set a different font size for the root element, you can add a CSS rule.

html { font-size: 20px; } h1 { font-size: 2rem; }

In the example above, the font size of the root element, <html>, is set to 20 pixels. All subsequent rem measurements will now be compared to that value and the size of h1 elements in the example will be 40 pixels.

One advantage of using rems is that all elements are compared to the same font size value, making it easy to predict how large or small font will appear. If you are interested in sizing elements consistently across an entire website, the rem measurement is the best unit for the job. If you're interested in sizing elements in comparison to other elements nearby, then the em unit would be better suited for the job.

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