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<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

Specification Status Comment
CSS Values and Units Module Level 3
The definition of '<frequency>' in that specification.
Candidate Recommendation Initial definition.

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

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support No support No support No support[1] No support
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support No support No support No support

[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.

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https://developer.mozilla.org/en-US/docs/Web/CSS/frequency