The official opacity tag is
This is part of the standards and is understood by all browsers released in the last couple of years.
For earlier Mozilla browsers use
For earlier Safari and Konqueror browsers use
These are also according to the standards since they have the appropriate standards extension qualifiers on the front.
For Internet Explorer you need to use
Note that this entry is not according to the standards since Microsoft omitted the -ie- off of the front that would identify it as a valid browser specific extension.
Also note that while the first three specify an opacity level between 0 and 1 the IE versionspecifies a level between 0 and 100.