Here is a very interesting page on comic book grammar and tradition. Written by Nate Piekos, a comic artist, letterer and font designer, it presents a series of ad hoc comic panels with text explaining how to convey particular styles, moods, sounds, and punctuation through speech balloons and related graphic devices. Piekos writes:
Comic book lettering has some grammatical and aesthetic traditions that are unique. . . . The majority of these ideas have been established by [publishers] Marvel and DC, but opinions vary from editor to editor
With 34 panels arranged in alphabetical order, the short supplementary notes cover a wide range of comic book language and expression, for example hyphenation, foreign languages, coughs, music, and telepathic speech.
Piekos explains such details as when to use lowercase, how to convey “off-camera” speech, and in what order to place the marks in an interrobang (!? or ?! – “It’s a loose rule that the question mark should come first”).
Sometimes these conventions shift. Of whispering (see image, reproduced with permission), he says:
Traditionally, whispered dialogue is indicated by a balloon with a dashed stroke. More recently accepted options are a balloon and dialogue in a muted tone (grayed-out), or with a lowercase font in conjunction with small dialogue/big balloon.
While reading comics, I have noticed some typographic patterns but never established whether they were standardised forms, for example the fact that emphasis is normally illustrated by a combination of bold and italics (and sometimes underlines), seldom if ever by either in isolation. I’m glad to have some of these observations and wonderings confirmed, and to have a host of others clearly summarised.
You can see them all here.
Interested readers are also encouraged to visit Gwillim Law’s history of grawlixes (aka obscenicons: taboo words represented by typographic symbols, which I previously linked to here), Ben Zimmer’s related commentaries at Language Log, and Doug Gilford’s Don Martin dictionary of Mad magazine sound effects.
Sentence first 
Thanks for pointing this out; it was a great way to wake up this morning!
For teachers- The Comic Book Project out of the Center for Educational Pathways is a great way engage students with comic langauge. http://www.comicbookproject.org/
Wow! Who knew that ? had to come before ! or Marvel Comics would reject the lettering?
On the other hand, there is no mention in the guide of the one most obvious difference between comic-book lettering and ordinary typesetting: all sentences end with exclamation points! Like this! (Apologies for shouting!)
Stan, I’m delighted with ‘interrobang’, which is new to me. Is there a word for several exclamation marks in succession?
Excellent it is the sort of thing we would have seen all the time and understood without thinking
Gabe: It was a pleasure. A good morning and good evening to you.
Sarah: Thanks for the link.
John: Most of them do, it’s true. I used to search for lines in comics that anticlimaxed with a lowly full stop. (No apology needed: I didn’t interpret your lines necessarily as shouted exclamations, more as dramatic ones.)
Mise: The interrobang also exists in combined form: ‽ Funny you should ask about multiple exclamation marks. I don’t know if there’s a standard term, but (as per this recent post) I’ve adopted Speculative Grammarian‘s phrase “multiple shriek stop“.
Jams: Yes, a lot of the devices and conventions are so intuitive as to be effectively invisible; it’s only when you focus on them that you see how much formal sense they make.
I never cease to be amazed at how much I learn here!
I remember, as a child, wondering at the interrobangs on the old Superman comics.
XO
WWW
It’s a mark I’ve always been fond of, WWW, though I don’t often use it. When I do, I order its constituents (!? vs. ?!) according to whether I want to stress the interrogation or the exclamation. To me, the second place is the stronger. Wikipedia has a nice page on it.
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I love this! I’m not a huge comic book fan, but I’ve read them in my day. I had no idea about all these rules for lettering and balloon style. But it certainly makes sense for providing clarity.
Glad you enjoyed it, Jan! Even in a narrative context as free as comics, it makes sense to have certain formal norms. They can always be tweaked or ignored, but the consistency they provide is important for the reader.