<frequency>
<frequency> CSS数据类型表示频率维度，例如语音的音高。目前它未在任何 CSS 属性中被使用。
语法
The <frequency> data type consists of a <number> followed by one of the units listed below. As with all CSS dimensions, there is no space between the unit literal and the number.
单位
可以使用以下单位：
备注：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 specified in the SI.
示例
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 an allowable <length>, it's an invalid <frequency>.
规范
| Specification |
|---|
| CSS Values and Units Module Level 4 # frequency |
备注：This data type was initially introduced in CSS Level 2 for the now-obsolete aural media type, where it was used to define the pitch of the voice. However, the <frequency> data type has been reintroduced in CSS3, though no CSS property is using it at the moment.
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