I've just found the solution of the problem : in the ezxml editor just use shift+space to create a nonbreaking space. The code used when the editor is desactivated is " ".
Some issues have also been made for that : http://issues.ez.no/IssueView.php?Id=13032 http://issues.ez.no/8494, I should have begun to search there !
Small note : don't hesitate to add a comment to the issues you found, giving more information on the issue based on your findings. Generally, this is a good practice, on any issue you may encounter.
Cheers,
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Nicolas Pastorino
Director Community - eZ
Member of the Community Project Board