For at least two centuries, it has been standard practice in the United States to place commas and periods inside of quotation marks. This rule still holds for professionally edited prose: what you’ll find in Slate, the New York Times, the Washington Post—almost any place adhering to Modern Language Association (MLA) or AP guidelines. But in copy-editor-free zones—the Web and emails, student papers, business memos—with increasing frequency, commas and periods find themselves on the outside of quotation marks, looking in. A punctuation paradigm is shifting.
Indeed, unless you associate exclusively with editors and prescriptivists, you can find copious examples of the “outside” technique—which readers of Virginia Woolf and The Guardian will recognize as the British style—no further away than your Twitter or Facebook feed. I certainly can — via redwolf.newsvine.com