Select the version of CSS to presume
The people responsible for defining CSS, W3, define what parts of CSS are stable, and what parts are in a state of flux. This simply reflects what the browsers are capable of processing correctly and consistently. Each snapshot is named for the year in which it was defined, and reflects the state of browsers at that time.
The CSS standards are regarded as extremely stable and correctly implemented, some are regarded as rather good (‘+’), and some are seen to be well on their way (‘++’), but most are seen as not yet sufficiently well implemented to be properly used. Unfortunately, the terminology used in the various snapshots is inconsistent, which is why ssc uses it own sarcastic terminology for the snapshots:
Below the snapshot selection is a full list of all CSS modules known to ssc. Many modules have more than one version, with higher numbered versions being the more recent, with more features, but less likelyhood of being regarded as stable and correctly implemented in all browsers.
The actual module level is confusing, a confusing in the published standards. CSS was originally defined to have levels 1, various levels 2, and a couple of fresh at the time modules were added at level three. It was explicitly stated that anything at levels one and two were part of those standards and indepedent modules start at level three. This did not happen. Many new modules have been published by the same organisation, W3, at levels 1 and 2, despite clearly and obviously not being part of the original levels 1 and 2. The result is confusion. All I can suggest is picking up a copy of the various specifications and working out for yourself what is meant.