Take a selection in the page and turn it into a blockquote

gistfile1.js

//
// Take a user's selected content in the page, as HTML, and
// place it inside a blockquote.
//
// Allows the possibility of automatically converting it
// to Markdown with toMarkdown.
//

// the selection in the document -- Selection object
var sel = document.getSelection();

// the Range object for the selection
var range = sel.getRangeAt(0);

// the Range object's contents as a DocumentFragment.
//
// if the selection is <p>foo [bar ba]z</p> then the fragment will
// automatically be contained in <p/> without doing anything.
var docFragment = range.cloneContents();

// create a <blockquote> to store the document fragment
var quote = document.createElement("blockquote");
quote.appendChild(docFragment);

// put the <blockquote> in a container to see its contents
var container = document.createElement("div");
container.appendChild(quote);

container.innerHTML
//-> <blockquote><p>bar ba</p></blockquote>

// with toMarkdown you can then convert this HTML into Markdown syntax
//
// https://github.com/domchristie/to-markdown
toMarkdown(container.innerHTML)

//-> > bar ba

// This will also give you <em> and <strong> accordingly.