First, remember that according to the HTML spec it’s not italics and bold, it’s emphasis and strong importance. I think what’s more at issue here is a contradiction between the HTML spec and SMD spec:
HTML
The em element represents stress emphasis of its contents… The level of stress that a particular piece of content has is given by its number of ancestor em elements… The em element also isn’t intended to convey importance; for that purpose, the strong element is more appropriate.
SMD
Markdown treats asterisks (*) and underscores (_) as indicators of emphasis.
So while HTML stresses that em and strong are completely different elements, SMD clearly conflates them and considers strong to be “strong emphasis”. I don’t see a clear path here.
- Using only * for strong and only _ for em would be more compliant with the HTML standard by separating the two elements. SMD does already allow for nesting of em and strong, so “strong emphasis” could still be conveyed by nesting.
- The existing behavior is more consistent with Markdown’s original (though technically incorrect) interpretation of em and strong. I think this is also pretty consistent with those elements’ usage on the net.
When it comes down to it, I think this is really an HTML5 problem. i, b, em, and strong are pretty high on the list of inconsistently used elements, so what should SMD do? Be a stickler and try to follow the HTML5 standard or go for ease-of-use and do what people usually intend?