For the time being you can disable the StripCssExpressions  filter:
    protected void Page_Load(object sender, EventArgs e)
    {
		RadEditor1.DisableFilter(Telerik.Web.UI.EditorFilters.StripCssExpressions);
    }

Alternative approach to fix the issue is removing the StripCssExpressions filter from the client-side filters collection, so that the server-side functionality could still sanitize the HTML content:

<telerik:RadEditor runat="server" ID="RadEditor1" OnClientInit="OnClientInit">
	
</telerik:RadEditor>

<script type="text/javascript">
	function OnClientInit(sender, args) {
		sender.set_contentFilters(sender.get_contentFilters() - Telerik.Web.UI.EditorFilters.StripCssExpressions);
	}
</script>