CSS <number> Data Type

The CSS <number> data type represents real numbers, possibly with a fractional component.

A valid <number> can include fractions, and it can also be immediately preceded by - (negative sign) or + (positive sign) to indicate a negative or positive value respectively. A <number> can also have an exponent composed of "e" or "E" and an integer.

So the following are all valid <number> values:

Example

When you see <number> (including the < and >) anywhere in the CSS specifications, this refers to the fact that the value can be any valid number.

For example, the official syntax for the cubic-bezier() function looks like this:

Which means that you must pass four arguments of type <number> to the function whenever you use it.

So you could do something like this:

Which will result in this:

<style>
  div {
    transition-property: width;
    transition-duration: 2s;
    transition-timing-function: cubic-bezier(.63,.05,.43,1.7);
    background: orange;
    color: white;
    width: 80px;
    margin: 10px;
    padding: 10px;
  }
  div:hover {
    width: 80%;
  }
</style>

<div>Hover...</div>

Official Definition

The official definition for the <number> type goes like this:

Number values are denoted by <number>, and represent real numbers, possibly with a fractional component.

When written literally, a number is either an integer, or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits and optionally an exponent composed of "e" or "E" and an integer. It corresponds to the <number-token> production in the CSS Syntax Module [CSS3SYN]. As with integers, the first character of a number may be immediately preceded by - or + to indicate the number's sign.

CSS Specifications

The <number> data type is defined in CSS Values and Units Module Level 3 (W3C Candidate Recommendation, 29 September 2016)
It is also defined in the Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification (W3C Recommendation 07 June 2011)
It is also defined in Cascading Style Sheets, level 1 (W3C Recommendation 17 Dec 1996)