As of Opera 13 your CSS works, see http://caniuse.com/word-break. But there is no support for this rule in Opera 12.
The better solution would be hypenation. But this works even less via CSS, see http://caniuse.com/css-hyphens.
So my best guess: You may want to use Javascript like https://code.google.com/p/hyphenator/ to add hyphens to your text.