The intended of interpreted of rule 11 is that you can’t have an unescaped
*at the beginning or end of the text that is inside*-delimited emphasis or strong emphasis. Sorry if that wasn’t clear enough from the wording. So for example**foo bar **can’t be interpreted as emphasized*foo bar *(Example 390). But Example 416 is okay because the unescaped*s come outside of the emphasis.
Ok, got it but then I don’t understand the behavior.
In
*****Hello*world****the first delimiter run is*****and the second is the lone*. Sum of the number of stars is 6, a multiple of 3, and the second one here can both open and close, so rule 9 is triggered.
Yes, that explains why the last star from the 1st delimiter run and the star of the 2nd run do not form an emphasis of Hello. No mentioned parser does that.
Once that is resolved, the parser then tries to use the middle star as an opener because the last delimiter has 4 stars. 4 + 1 = 5 is not divisible by 3. All the mentioned parsers follow that and produce the emphasis of world.
However why then dingus/cmark does not try to continue using the stars of the 1st delimtier run and the 3rd delimiter run for additional emphasis/strong emphasis? The 2nd delimiter plays no role here anymore. None of the 1st and 3rd delimiter runs can be both an opener and a closer. Therefore the rule 9 should not apply anymore in this step.