Re: le but ?

Voilà ce qu'en dit (le brouillon de) la spec officielle :

Any text following the leading "```" of a preformat toggle line which toggles preformatted mode on MAY be interpreted by the client as "alt text" pertaining to the preformatted text lines which follow the toggle line. Use of alt text is at the client's discretion, and simple clients may ignore it. Alt text is recommended for ASCII art or similar non-textual content which, for example, cannot be meaningfully understood when rendered through a screen reader or usefully indexed by a search engine.