As word-wrap: break-word does not work well when the width is not fixed (and even less in ), Stack Exchange adds some invisible Unicode markers, which browsers use to find possible positions for line breaks. But: those invisible markers are still there when the visitor copies the text, which might be bad. More details on Meta, in https://meta.stackexchange.com/questions/170970/occasionally-the-unicode-character-sequence-u200c-u200b-zwnj-zwsp-is-insert/.
Instead of Unicode magic, inserting every few characters seems not to have a bad effect when copying the resulting text. This works for me in Chrome 25, Firefox 19, Safari 6, Internet Explorer 9 (in Windows 7 on Parallels on a Mac; simulating IE8 and IE7 works too), stock browser and Chrome on Android 4.1, and Safari on iOS 6.1.2, and is probably also supported in Opera. See this JS Bin example.
Some day, will be the solution, but not today: though IE7 supported it, IE8 and IE9 don't.