Comics, like most storytelling mediums, is a balancing act between art and fiction. The problem between the two is that one is filled with structure and rules while the other relishes in breaking them. But storytelling is not an exact science. It’s a feeling, an instinct, a writer/creator’s intuition as to what the product in their head should look like. And in that regard, the formal rules of grammar and writing clash with the free-wheeling spirit of art and story.
This particularly happens a lot with dialogue and punctuation. Things like ellipses, commas, and strict rules that should be applied in order for a sentence to be dramatically correct. For a prose novel, these rules are often followed to the letter (no pun intended) in order to avoid criticism. In fact, novels with errors such as these are often looked at as inferior or subpar to those that are perfect.
But comics are different. The words aren’t the only thing telling the story. The art is the primary factor that is communicating what is going on in the scene. In fact, the majority of language seen in a comic will be spoken dialogue. That means it may include certain errors or dialects in order to properly convey characterization.
Even more than that though, dialogue on the page should reflect a feeling or emotion. Comic writers are very particular in how they want their dialogue absorbed. Punctuation goes a long way to dictate the pacing and emphasis that a reader will experience. Taking this into account, comics can bend and break the rules of grammar more than traditional fiction. Of course, there are exceptions to this. Generally speaking, voice over narration and caption boxes might follow strict grammar guidelines, but for the most part, comic reading lends itself more towards the spirit of grammar rather than the rules of it.