In HTML5, <blockquote> content "must be quoted from another source". So that's pull quotes completely out of the window, then.
Heydon rightly makes the distinction between block quotes, which (supposedly) quote another source, and pull-quotes, which highlight excerpts from the article you're reading. I was surprised to learn that <blockquote> elements should not be used for pull-quotes.
...the term "block quotation" precedes the <blockquote> element and the concept of block-level HTML elements. The Chicago Manual Of Style recommends block quotations are over 100 words in length, for example.
Different versions of different specifications agree the citation cannot be inside the <blockquote> element.
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