I think using quote blocks for excerpts would be the best format. Here's why:
This is a long excerpt. It's not really an important excerpt, but at least you can read all the way to the end of it, because quote blocks soft wrap the text and that means that you can see the entire thing, without having to scroll unnecessarily. Also, you can format this text as well, so you can include any original emphasis or formatting.
This is also a long excerpt It's not an important sentence either, but it doesn't wrap. Here are some more characters so you get the scrollbar at the bottom of this excerpt.
Well, it can wrap, but I have to put hard wraps in. Try viewing this on mobile.
You also can't format this, so **you need to use pseudo-formatting**. Also, fixed
width font is usually harder for people to read.
Also, bonus downside of code blocks for mobile clients: potentially less width, and unexpected syntax highlighting!