Please note, this is a STATIC archive of website developer.mozilla.org from 03 Nov 2016, cach3.com does not collect or store any user information, there is no "phishing" involved.
Is a keyword indicating to use the browser-defined default spacing between columns. The specification, which most modern browsers follow, recommends that this be equal to a length of 1em.
<length>
Is a <length> value defining the size of the gap between columns. It must not be negative, but may be equal to 0.