It is common to reference a creative work within a paragraph. HTML provides the <cite> tag to do this semantically, defining a “creative work” quite broadly:
Creative works include a book, a paper, an essay, a poem, a score, a song, a script, a film, a TV show, a game, a sculpture, a painting, a theatre production, a play, an opera, a musical, an exhibition, a legal case report, a web site, a web page, a blog post or comment, a forum post or comment, a tweet, a written or oral statement, etc.
By default browsers render <cite> in italics. References to creative works are altogether different from emphasis (<em>), and <i> does not capture the semantic meaning.
I have not come across any Markdown flavours that contain special syntax for <cite>; this is surprising as writing the HTML by hand can become tedious. MultiMarkdown contains citation syntax, but that is for bibliographies.
Proposals:
(1) It is common to use double quotes to reference shorter creative works, such as an article title, saving the use of italics for longer creative works such as book titles. Since the use of the double quote is already common for referencing creative works, this character could be reused in Markdown by surrounding the text in double double quotes. For example:
""Tractatus Logico-Philosophicus"" was first published in 1921.
which would translate into the following HTML:
<cite>Tractatus Logico-Philosophicus</cite> was first published in 1921.
(2) Alternatively, cited works could be referred in a similar way to Markdown Extra’s abbreviations. Writers could define “Tractatus Logico-Philosophicus” once in the document as a creative work and any references to it would be automatically wrapped in <cite></cite>. Note, this approach could be problematic if the creative work had common name such as the television show “Girls”. We may not want every instance of “Girls” within the document to reference the show, so explictly referencing the work (as in the first proposal) is safer.