4.6. CSS2 Units
In addition to what we've covered in CSS2.1, CSS2 contains a few extra units, all of which are concerned with aural style sheets (employed by those browsers that are capable of speech). These units were not included in CSS2.1, but since they may be part of future versions of CSS, we'll briefly discuss them here:
Used to define the position from which a given sound should originate. There are three types of angles: degrees (deg), grads (grad), and radians (rad). For example, a right angle could be declared as 90deg, 100grad, or 1.57rad; in each case, the values are translated into degrees in the range 0 through 360. This is also true of negative values, which are allowed. The measurement -90deg is the same as 270deg.
Used to specify delays between speaking elements. They can be expressed as either milliseconds (ms) or seconds (s). Thus, 100ms and 0.1s are equivalent. Time values cannot be negative, as CSS is designed to avoid paradoxes.
Used to declare a given frequency for the sounds that speaking browsers can produce. Frequency values can be expressed as hertz (Hz) or megahertz (MHz) and cannot be negative. The values' labels are case-insensitive, so 10MHz and 10Mhz are equivalent.
The only user agent known to support any of these values at this writing is Emacspeak, an aural style sheets implementation. See Chapter 14 for details on aural styles.
In addition to these values, there is also an old friend with a new name. A URI is a Uniform Resource Identifier, which is sort of another name for a Uniform Resource Locator (URL). Both the CSS2 and CSS2.1 specifications require that URIs be declared with the form url(...), so there is no practical change.