This panel is part of the Settings Dialogue. It controls CSS validation.
Unlike HTML. CSS is not defined by one huge standard, rather by many small ones. Here, you can choose which versions of CSS to acknowledge.
Fortunately, W3 irregularly releases documents saying which versions of what parts of CSS are supported by the browsers; which parts of CSS are stable, and which are in a state of flux. This reflects what the browsers of the time are capable of processing correctly and consistently. Each snapshot is named for the year in which it was defined.
The CSS standards are regarded as extremely stable and correctly implemented, some are regarded as rather good (‘+’), some are seen to be well on their way (‘++’), and a few more are seen as worth a glance (‘+++’). Most, though, are 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 instead:
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 confusion originating 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 3. 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 newer modules have been published by the same organisation, W3, at levels 1 and 2, despite clearly and obviously not being part of the original, and still activately used, levels 1 and 2. The result is a thorough mess. All I can suggest is picking up a copy of the various specifications and working out for yourself what is meant.
Many of those post CSS 2 specifications were significantly revised, giving version 4, etc., all the way up to, in one case, version 7. Selection a specific version of CSS in SSC selects all version of CSS extensions, up to and including that selected version.
Normally, CSS files have the extension .css. However, some sites are weird, and use something else. Here, you can take into account such bizarritudes.