@moreteaActually for most, those are essentially inconsequential when reading as I read enough of both to easily parse the differences. I do focus on it while reading, just to be sure that what I do write isn't misinterpreted, though that's not a concern when reading casually.
American and classical British spellings are pretty well set, as are their various punctation marks, so they're easily anticipated. Though in many cases, as has already been pointed out, the American uses of many of those have become the norm simply because of the sheer volume of material (books, film, tv, online series, etc.) so in those cases, many Brits simply choose the American styles simply for simplicity and to access a broader market.
But it has never provided any errors in reading most English literature. I have a much harder time with Indian (the country) stories written in English, simply because of their different perspectives, yet that varies by region anyway.
Perhaps that's also why I'm not quite as precise with my terms, as I essentially don't notice many of them while reading. There's a big difference between regional spellings, idioms (sayings) or particular phrasings, yet it's mostly the different ways of writing that most throw me.