This is a follow-up.
You can achieve your goal by overwriting the _nodeAttributesArray of RadEditorNodeInspector which contains the definitions of each node panel, e.g.
<telerik:RadEditor ID=
"RadEditor1"
runat=
"server"
>
<Content><div style=
"color:red;background-color: yellow; border: 1px solid red; width:120px; height:20px;"
>test</div></Content>
</telerik:RadEditor>
<script type=
"text/javascript"
>
Telerik.Web.UI.Editor.Modules.RadEditorNodeInspector.prototype._nodeAttributesArray =
//Contains definitions for each node panel
{
DIV: [
"width"
,
"borderColor"
,
"bgColor"
,
"className"
,
"height"
,
"border"
],
TABLE: [
"width"
,
"borderColor"
,
"cellSpacing"
,
"bgColor"
,
"className"
,
"SetTableProperties"
,
"height"
,
"cellPadding"
,
"align"
,
"border"
],
TH: [
"width"
,
"borderColor"
,
"bgColor"
,
"className"
,
"SetCellProperties"
,
"height"
,
"align"
,
"noWrap"
,
"border"
],
TD: [
"width"
,
"borderColor"
,
"bgColor"
,
"className"
,
"SetCellProperties"
,
"height"
,
"align"
,
"noWrap"
,
"border"
],
TR: [
"width"
,
"className"
,
"height"
],
A: [
"href"
,
"className"
,
"LinkManager"
,
"title"
,
"target"
],
IMG: [
"width"
,
"borderColor"
,
"className"
,
"SetImageProperties"
,
"height"
,
"align"
,
"border"
,
"alt"
],
INPUT: [
"NAME"
,
"width"
,
"height"
,
"id"
,
"title"
,
"className"
,
"value"
],
FORM: [
"className"
,
"width"
,
"height"
,
"NAME"
,
"action"
,
"id"
],
TEXTAREA: [
"className"
,
"width"
,
"height"
,
"NAME"
,
"id"
,
"rows"
,
"cols"
]
}
</script>