This span is font-size:20px; Px (pixel) is an absolute unit. A pixel represents a single dot on a monitor. This sizing will vary from screen to screen because screens with different resolutions display different amounts of pixels per inch

This span is font-size:16px;

This span is font-size:31pt; A pt (point) is an absolute unit. A pt i 1/72 of an inch. Since this is 32 points its half an inch tall.

This span has no styling apart from color

This span is font-size:1em; A em is a relative unit. Notice that its the same size as the font with no styling.

This span is font-size:2em; A em is a relative unit. Notice that its twice the size of the font with no styling.

This span has no styling apart from color

This span is font-size:100%; A percent is a relative unit. Notice that its 100% of the same size as the font with no styling.

This span is font-size:200%; A percent is a relative unit. Notice that its 200% of the same size as the font with no styling.