This tech­nique works with text in <pre> tags and with long strings that aren’t bro­ken up by spaces (like this_is_a_very_long_string). I strug­gled with this at work for a bit, then hit Google and found the solu­tion at this infor­ma­tive site.

Here’s the code:

pre {
 white-space: pre-wrap;       /* css-3 */
 white-space: -moz-pre-wrap;  /* Mozilla, since 1999 */
 white-space: -pre-wrap;      /* Opera 4-6 */
 white-space: -o-pre-wrap;    /* Opera 7 */
 word-wrap: break-word;       /* Internet Explorer 5.5+ */
}

Just sub out the pre for the selec­tor you’re work­ing with if you need this for long strings.