Most browsers will accept the style opacity: .50;
However, IE requires filter: alpha(opacity=50);
Adding this into an inline style attribute manually and then saving removes the style from the attribute. Adding it to a CSS style will persist over a save, but as soon as the CSS editor is used, the stylle is removed and needs to be re-added.
What should happen is when transparency is set, it should set both the opacity and the filter: alpha(...) styles and persist the styles over edits.
it might be worthwhile to also set the -moz-opacity style as well(works like the opacity style, but is specific to some older mozilla browsers).