Only use block-level elements in blockquotes:

Something I see quite a bit of around the Web is the blockquote element being slightly abused. [...] That’s right, the blockquote element is not allowed to have text or inline elements as direct descendants.

That’s the great thing about this industry: there’s always more to learn, even when it comes to the basics. Putting raw text in a <blockquote> tag probably became a habit for me 10 years ago.