<frequency>

The <frequency> CSS data type represents a frequency dimension, such as the pitch of a speaking voice.

Syntax

A frequency consists of a <number> followed by a unit. As with all CSS dimensions, there is no space between the unit literal and the number.

Units

Hz

Represents a frequency in Hertz. Examples: 0Hz, 1500Hz, 10000Hz.

kHz

Represents a frequency in kilohertz. Examples: 0kHz, 1.5kHz, 10kHz.

Although the number 0 is always the same regardless of unit, the unit may not be omitted. In other words, 0 is invalid and does not represent 0Hz or 0kHz. Though the units are case-insensitive, it is good practice to use a capital H for Hz and kHz, as in the SI, Hertz being a proper noun.

Examples

Valid frequency values

12Hz     Positive integer
4.3Hz    Non-integer
14KhZ    The unit is case-insensitive, though non-SI capitalization is not recommended.
+0Hz     Zero, with a leading + and a unit
-0kHz    Zero, with a leading - and a unit

Invalid frequency values

12.0     This is a <number>, not an <frequency>, because it is missing a unit.
7 Hz     No space is allowed between the number and the unit.
0        Although unitless zero is allowed for <length>s, it's invalid for <frequency>s.

Specifications

This data type was initially introduced in CSS Level 2 for the now obsolete aural media group, where it was used to define the pitch of the voice. This has been deprecated since then, but the <frequency> data type has been reintroduced in CSS3, though no CSS property is using it at the moment.

Browser compatibility

[1] Some versions of Opera may have (partial) support for the obsolete aural media group, and through the pitch property support for the <frequency> data type.