Markdown does not contain syntax for the
<i> tag. Although this was a presentational tag in older versions of HTML, it has been redefined to have semantic meaning in HTML5. From the spec:
The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts.
I read through the recently published draft specification for Markua (the replacement for Leanpub Flavored Markdown). It contains some thought provoking points and ideas, some of which could be considered for CommonMark. The draft spec for Markua adds Markdown-style syntax for alternative voice. It does this by redefining the use of the underscore surrounding text for
<i>, while keeping asterisks for emphasis (
This is some _italicized with an old-school but now semantic i tag_ text.
<p>This is some <i>italicized with an old-school but now semantic i tag</i> text.</p>
This breaks compatibility with Markdown, but is understandable. This may not be the best approach for CommonMark, given that authors of older Markdown documents may have intended the underscores to mean emphasis rather than alternative voice. I thought it would be worth looking at, at least.
Since alternative voice is quite commonly used in prose, Markdown-style syntax of some kind would be beneficial as a CommonMark extension. Besides Markua, I don’t know of any flavours that have specified syntax for it.