org/TR/CSS21/syndata.html#vendor-keywords
The Ultimate CSS Reference 368
'-' + vendor specific identifier + '-' + meaningful name
'_' + vendor specific identifier + '-' + meaningful name
This approach allows any vendor-specific extension to coexist with any future (or
current) CSS properties without causing conflicts because, according to the W3C
specifications, a CSS property name will never begin with a dash or an underscore:
???An initial dash or underscore is guaranteed never to be used in a property or
keyword by any current or future level of CSS. Thus, typical CSS implementations
may not recognize such properties, and may ignore them according to the rules for
handling parsing errors. However, because the initial dash or underscore is part of
the grammar, CSS2.1 implementers should always be able to use a CSS-conforming
parser, whether or not they support any vendor-specific extensions.???2
A number of extensions are known to exist. Their prefixes are outlined in Table
16.
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