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.
…But the main reason is that the British way simply makes more sense. Indeed, since at least since the 1960s a common designation for that style has been “logical punctuation.
By far the biggest fount of logical punctuation today is Wikipedia, which was started by two Americans but whose English-language edition is by and for all English-speaking countries. The site’s style guide notes that “logical punctuation … is used here because it is deemed by Wikipedia consensus to be more in keeping with the principle of minimal change.” That is, if you put a period or comma inside quotation marks, you are wrongly suggesting that the period or comma is part of the quoted material, and thus you have “changed” it.
I’ve been doing this for quite a while now, and I honestly had no idea I was part of a community of like-minded individuals! I knew what the official rule was about punctuation and quotation marks, but since I’m just a ruffian blogger, a cyber-street urchin, I figured I would do what made obvious fucking sense to me and damn the style guides and rulebooks.