The <integer>CSS data type denotes an integer number, positive or negative. There isn't any associated unit with the value. An integer consists of one or several decimal digits, 0 to 9, optionally preceded by one single + or - sign.
The <length>CSS data type denotes distance measurements. It is a <number> immediately followed by a length unit (px, em, pc, in, mm, …). Like for any CSS dimension, there is no space between the unit literal and the number. The length unit is optional after the <number>0.
The <percentage>CSS data types represent a percentage value. Many CSS properties can take percentage values, often to define sizes in terms of parent objects. Percentages are formed by a <number> immediately followed by the percentage sign %. Just as is the case with all other units in CSS, there isn't a space between the '%' and the number.
The <ratio>CSS data type, used for describing aspect ratios in media queries, denotes the proportion between two unitless values. It is a strictly positive <integer> followed by a slash ('/', Unicode U+002F SOLIDUS) and a second strictly positive <integer>. There may be spaces before and after the solidus.
The <resolution>CSS data types, used in media queries, denotes the density of pixels of an output device, its resolution. It is a <number> immediately followed by a unit of resolution (dpi, dpcm, ...). Like for any CSS dimension, there is no space between the unit literal and the number.
The <string>CSS data type represents a string. It is formed by a Unicode characters delimited by either double (") or single (') quotes. A double quoted string cannot contain double quotes unless escaped using a backslash (\). The same practice applies for single quoted strings, they cannot contain single quotes unless escaped using a backslash (\). The backslash character must be escaped to be part of the string.
The <angle>CSS data type represents angle values. Positive angles represent clockwise angles, negative angles represent counterclockwise angles. Its syntax is a <number> data type immediately followed by the unit (deg, grad, rad or turn). Like for any CSS dimension, there is no space between the unit literal and the number.
The <basic-shape> type can be specified using basic shape functions. When using this syntax to define shapes, the reference box is defined by each property that uses <basic-shape> values. The coordinate system for the shape has its origin on the top-left corner of the reference box with the x-axis running to the right and the y-axis running downwards. All the lengths expressed in percentages are resolved from the used dimensions of the reference box.
The <gradient>CSS data type denotes a CSS <image> made of a progressive transition between two or more colors. A CSS gradient is not a CSS <color> but an image with no intrinsic dimensions; that is, it has no natural or preferred size, nor a preferred ratio. Its concrete size will match the one of the element it applies to.
The <image>CSS data type represents a 2D image. There are two kinds of images in CSS: plain static images, often referenced using a URL, and dynamically-generated images like gradients or representations of parts of the tree.
The <number>CSS data type represents a number, either integer or fractional. Its syntax extends the one of the <integer> data value. To represent a fractional value, add the fractional part — a '.' followed by one or several decimal digits — to any <integer> data value. Like for <integer> data type, there isn't any unit associated to a <number>, which is not a CSS dimension.
The <time>CSS data type denotes time dimensions expressed in seconds or milliseconds. They consists of a <number> immediately followed by the unit. Like for any CSS dimension, there is no space between the unit literal and the number.
The <timing-function>CSS data type denotes a mathematical function that describes how fast one-dimensional values change during transitions or animations. This in essence lets you establish an acceleration curve, so that the speed of the animation can vary over its duration. These functions are often called easing functions.
The <transform-function> CSS data type denotes a function applied to an element's representation in order to modify it. Usually such transform may be expressed by matrices and the resulting images can be determined using matrix multiplication on each point.