This page does not list all HTML meta tags.
Meta tags add value to a web page. Visitors do not see the meta tags, but they can be the difference in search results and social linking. The end result is that a website can tweak its online presence using meta tags.
NOTE: The custom meta tags are unique to the @Gyrospring network, so they probably don't work with any social linking logic. The result in keeping custom social meta tags is I will be able to list the stuff one day. I'm not even really sure why this has not been instituted, you could have thousands of social groups instituting incompatible things.
Common meta tags that should be included with every web page.
@Gyrospring has created custom meta tags for social linking. These tags are probably not supported by most, if not all, websites. These tags were created/implemented as a generic solution for all social crawlers.
Charset is covered in the Document Encoding page.