CSS offers a number of different units for expressing length what are they?

Answer 1

Many CSS properties take values such as font-size, margin, line-height, etc. A value can be negative in some cases. Some properties may restrict the length value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

CSS offers a number of different units for expressing length. Some have their history in typography, such as point (pt) and pica (pc), others are known from everyday use, such as centimeter (cm) and inch (in). And there is also a unit that was invented specifically for CSS: the pixel (px).

CSS also defines a set of other length units, all of which, including the previously mentioned ones, fall in one of two categories: absolute units or relative units.

For more information http://tympanus.net/codrops/css_reference/length/

Answer 2

Many CSS properties take "length" values, such as width, margin, padding, font-size, border-width, etc.

Length is a number followed by a length unit, such as 10px, 2em, etc.

A white space cannot appear between the number and the unit. However, if the value is 0, the unit can be omitted.

For some CSS properties, negative lengths are allowed.

There are two types of length units: relative and absolute.

Relative Lengths

Relative length units specify a length relative to another length property. Relative length units scales better between different rendering mediums.

Unit Description

em  Relative to the font-size of the element (2em means 2 times the size of the current font) 

ex  Relative to the x-height of the current font (rarely used)

ch  Relative to width of the "0" (zero)

rem Relative to font-size of the root element

vw  Relative to 1% of the width of the viewport

vh       Relative to 1% of the height of the viewport

vmin    Relative to 1% of viewport's smaller dimension

vmax    Relative to 1% of viewport's larger dimension

%    

px  pixels (1px = 1/96th of 1in)

Absolute Lengths

The absolute length units are fixed and a length expressed in any of these will appear as exactly that size.

Absolute length units are not recommended for use on screen, because screen sizes vary so much. However, they can be used if the output medium is known, such as for print layout.

Unit Description

cm  centimeters 

mm  millimeters 

in  inches (1in = 96px = 2.54cm)

pt  points (1pt = 1/72 of 1in)

pc  picas (1pc = 12 pt)