I tried it myself...:
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I'm not sure this is a good idea. We have the .net tag to talk about the general field of .NET, as well as version-specific tags (such as .net-4.5) for items that are specific (or more likely, introduced) in that specific version.
The css tags are no different. There are times when you are speaking about CSS in general (or, the version that is most prevalent/widely-adopted at the time), but there are times when you want to speak about a specific version; this situation is when something isn't fully baked/just released so you can differentiate the technologies when an influx of new questions are going to come in on that new technology.
Going back and removing the version-specific tags (you're suggesting that css is version one and that it be merged into version two) generally isn't a good idea. If anything, there might be people still using the old technology (depending on what we're talking about), and still need to differentiate between the two.