How to get autocomplete of CSS3 properties on Eclipse inside the <ui:style> tag of GWT UiBinder?

Question

I can enable autocomplete of CSS3 properties for .css and .gss files on Eclipse by following this answer by Nitin DahyabhaiFriend:

The default profile in Luna is still CSS2. Bring up the Properties dialog for your web project and you can change the default CSS profile for that project on the Web Content Settings page. If the CSS file is not in a web project, or if you don't want to use the project default, you can also set it using the file's Properties dialog (also Web Content Settings page).

... but that doesn't enable the autocompletion of CSS3 properties (such as background-position, border-radius and so on) inside *.ui.xml files (GWT UiBinder). For example:

<ui:style>
    .randomStuff {
        background-repeat: no-repeat;
        background-position: center;
        background-s <ctrl+space... no luck!>
    }
</ui:style>

How can I enable autocomplete of CSS3 inside those files?