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• point (pt)
• pixel (px)
• percent (%)
• em
• root em (rem)
• viewport width (vw)
• viewport height (vh)
• vw or vh, whichever is smaller (vmin)
• vw or vh, whichever is greater (vmax)

• default typographic sizes are based on a scale of 16px body copy.

• pt. Points are 1/72 of an inch.
• px. Pixels are 1/96th of an inch. According to the CSS specification, "The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length." With high-DPI devices, such as the 2x or even 3x Retina screens on iPhones, iPads, and some Macs, we need to understand the difference between a hardware pixel and the CSS reference pixel. For details, see The CSS Layout Workshop by Rachel Andrew.

Go deeper: view the W3C specification for absolute units.

• em. The width of a letter "m." em is relative to the font size of its nearest ancestor.
• rem. The "root em" is relative to the font-size of the html, or root, element.
• %. Percents are always relative to another value.

Go deeper: view the W3C specification for relative units.

The units below are relative to the viewport—not to a reader's default font size.

• vw. 1% of the width of the initial containing block.
• vh. 1% of the height of the initial containing block.
• vmin. The smaller of vw or vh.
• vmax. The larger of vw or vh.

Go deeper: view the W3C specification for viewport relative units.