To italicize (punctuation) or not to italicize, that is the question.
The Rule (According to CMS 6.3): Punctuation should appear in the same font or typeface as the general body text of a document. So if you have a roman sentence that contains an italicized word followed by a comma, the comma should appear in roman.
Example 1: He’d lent his favourite film, City of Lost Children, to his mom, but she didn’t seem to like it.
Seems straightforward enough, but does this work all the time? Not quite…
The Exception to the Rule: The exception to this rule applies to punctuation that properly belongs to the italicized word. In this case the punctuation should be in italics, too.
Example 2: When I’m in Manhattan, should I go see Who’s Afraid of Virginia Woolf?
Chicago’s rule here is logical and gives readers helpful information (aha, the question mark is part of the title), which makes it a good default approach to dealing with italics and punctuation.
Alternate Rule: That said, there are publications and organizations whose house style will dictate that all punctuation after an italic word should also be italic. This is usually prescribed for aesthetic reasons about how the italic word looks with the punctuation that follows, and it’s an easy rule to follow.This rule can create some infelicities, though, like paired commas having different formatting. I can show you by applying the alternate rule to example 1 from above:
Example 3: He’d lent his favourite film, City of Lost Children, to his mom, but she didn’t seem to like it.
Aesthetically the comma fits nicely with the n at the end of Children; however, the paired commas that set off the name of the movie no longer match one another.
So What Rule Should You Follow?
As you may already have guessed, I prefer to follow Chicago’s rule in this matter because of its logic. Nevertheless, because consistency trumps almost everything else in editing, every day at work I apply the alternate rule, which is house style at the publisher I work for. You, too, can decide which style rules you prefer to apply to your own work, but you’ll also have to be prepared to fall into line with house styles whenever you run into them.