CSS has two forms of measurement, absolute and relative.

rem, vh, vw are CSS3 measurement units, supported by modern browsers.

1em is the width of the m character in the default font set for that browser. (ex is the height of the x character in a font). rem allows you to set a root font size for size for the entire page, with sizes for elements relative to this measurement:

1. html { font-size: 62.5%; }
2. body { font-size: 1.4rem; }
3. h1 { font-size: 2.4rem; }

(The measurements above are used because they map exactly to pixels: with a base font-size of 62.5% for a page, the body size maps to 14 pixels, and h1 to 24 pixels.)

The viewport is the browser window: vh, vw and vm can scale elements to its current size. This could be used to restrict the upper scaled size of images, for example:

1. img { max-width: 90%vw; }

ch is the equivalent of em, but applied to the width 0 numeral of the chosen font.

gd relates to the CSS3 grid; unfortunately, no browser yet supports the grid module.

You never need to specify units when declaring a value of 0: zero centimeters is the same as 0 pixels. In all other cases, units of measurement should be declared 2em, 2px, etc, not 2.

With the exception of pixels, all systems of measurement can take floating point values: 2.25em, 5.3cm, etc.

Generally speaking, CSS written for screen display devices should use relative units (percent, em and ex), with few exceptions (such as bitmapped images and elements that refer to bitmapped images).