HTML and CSS Reference
A frequency value ( <frequency> ) is expressed as a non-negative <number> followed imme-
diately by a frequency unit. There are two types of frequency units: hertz ( Hz ) and kilohertz
( kHz ). The unit identifiers are case-insensitive, so 6kHz and 6khz are equivalent. As of this
writing, frequency values are only used with aural styles, which are not well supported.