If you have a long word, e.g. Supercalifragilisticexpialidocious, and you put it inside a block (e.g. a <div>) with a maximum width and word-wrap set to break-word, the word will break at the maximum width as expected. However if you attempt the same thing with a XUL <description> or <label> element then the word will not break unless you additionally override the minimum width. If you interpose a <div> between the <description>/<label> and the text you can of course set a maximum width on the <div>, however setting a maximum width on the <description>/<label> still does not work. By comparison, setting a maximum width on an outer nested <div> does work. As a side note, overriding the minimum width on the container element does not work at all except in one case: a <vbox> containing a <vbox> with both minimum and maximum width and align="start" containing a <description>/<label>.

Neil, could you attach your testcases here?

Key: red: incorrect; would wrap if string contained spaces yellow: correct, but only through abuse of min-width green: correct; spaces don't change wrapping behaviour