Markdown Emphasis Issues in CJK: Analyzing CommonMark's Delimiter Rules

Summary

As Markdown has become the standard for LLM outputs, we are now forced to witness a common and unsightly mess where Markdown emphasis markers (**) remain unrendered and exposed, as seen in the image. This is a chronic issue with the CommonMark specification---one that I once reported about ten years ago---but it has been left neglected without any solution to this day. The technical details of the problem are as follows: In an effort to limit parsing complexity during the standardization process, CommonMark introduced the concept of "delimiter runs." These runs are assigned properties of being "left-flanking" or "right-flanking" (or both, or neither) depending on their position. According to these rules, a bolded segment must start with a left-flanking delimiter run and end with a right-flanking one. The crucial point is that whether a run is left- or right-flanking is determined solely by the immediate surrounding characters, without any consideration of the broader context. For instance, a left-flanking delimiter must be in the form of **<ordinary character>, <whitespace>**<punctuation>, or <punctuation>**<punctuation>. (Here, "ordinary character" refers to any character that is not whitespace or punctuation.) The first case is presumably intended to allow markers embedded within a word, like **마크다운**은, while the latter cases are meant to provide limited support for markers placed before punctuation, such as in 이 **"마크다운"** 형식은. The rules for right-flanking are identical, just in the opposite direction. However, when you try to parse a string like **마크다운(Markdown)**은 using these rules, it fails because the closing ** is preceded by punctuation (a parenthesis) and it must be followed by whitespace or another punctuation mark to be considered right-flanking. Since it is followed by an ordinary letter (은), it is not recognized as right-flanking and thus fails to close the emphasis. As explained in the CommonMark spec, the original intent of this rule was to support n...