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”).
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.