David Crystal’s recent book Making a Point has a rundown of the changes and a very clear framework for discussing them. I’m not going to do it justice, especially because I don’t remember every bit of it. But:
- There has been a tussle in the history of English punctuation from the invention of printing on, between punctuating according to syntax, and punctuating according to meaning. The former would never allow you to put a comma between a subject and a verb; the latter will let you put a comma after anything, so long as it’s long enough and a single syntactic constituent.
- Extreme meaning-based punctuation is why renaissance English seems so random. Extreme syntax-based punctuation is why 18th century English seems so mannered.
- We still rarely put a comma between a subject and a verb; but commas are now much more about meaning than they were two centuries ago.
- Big complicated multipart sentences are out of favour. The semicolon has suffered from that change in fashion.
- Punctuation online, and in texts, is newly creative, and is establishing its own distinct norms.