I just wonder, why after so much testing not one post summarizes all known formats and provides the needed codes.

Some are well documented, but don't list all options. Some list a lot of options, but don't show the codes. And some are only test suits and still provide codes that don't work.

Well, this post may also not be complete, because there are still some questions. But it assembles all available knowledge from this thread and some further testings.

=​=Headline=​= (2 equals signs) soley on a line gives
Headline
'​'italic'​' (2 apostrophes) gives italic.
'​'​'bold'​'​' (3 apostrophes) gives bold.
*​*masked*​* (2 asterisks) gives masked and reveals the text when hovered with the mousepointer.
Italic, bold and masked can be nested.

<​-​- (less-than sign + 2 hyphens) combine to ← (left arrow)
-​-​> (2 hyphens + greater-than sign) combine to → (right arrow)
-​- (2 hyphens) combine to – (short dash)
-​-​- (3 hyphens) combine to — (long dash)

>at the beginning of a line gives a quote.
​>​> with the number of an existing post gives a link to that post, like >>5708, where 40 hyphens resulted in a horizontal line. It seems, that this is not longer working.

If someone knows how to create the centered Headline in >>5597, the information would be appreciated. The same applies to other formatting codes.