Purpose: These attributes are intended for use where the marker of ordered list items itself conveys meaning to the content of the list. Typically this is the case when list items are intended to be referred to via this marker. This is in contrast to CSS list-style-type which is intended where the choice of marker carries no significant purpose. (Though I did see it recommended for forum-wide localization, which is pretty great!)
Background: While attempting to quote an official reference document, I discovered an apparent limitation in list formatting which prevents doing so accurately. Quite often, list markers are purely stylistic. However, they are occasionally used to facilitate reference in list-heavy documents. For example, in official and legal documents, a list item (or sub-item) might be referred to by list marker: “Official Document Article I, section 1(a)(i)”
For Markdown, there’s no mention in the CommonMark specification, and indeed it specifies only digits, 0-9. But while most interpreters don’t permit letters for lists(including markdown-it), a couple do:
I’m uncertain of whether a standard method is available in BBCode, to the extent anything is standard in BBCode. I was unable to find anything more advanced than starting at an arbitrary digit. BBCode list tags don’t appear used by Discourse, anyway.
With HTML and HTML+style, neither produce the correct result: