<script>
    function OnKendoWidgetInitializing(sender, args) {
        var series = args.series[0]
        series.highlight = series.highlight || {};
        series.highlight.inactiveOpacity = 0.2
    }
</script>
<telerik:RadHtmlChart ID="RadHtmlChart5" runat="server" BorderColor="Fuchsia" BorderStyle="Solid" BorderWidth="1px">
    <ClientEvents OnKendoWidgetInitializing="OnKendoWidgetInitializing" />
    <PlotArea>

        <Series>
            <telerik:PieSeries StartAngle="90">
                <%-- This is how it should be configured after the feature request --%>
                <%--<HighlightAppearance InactiveOpacity="0.2" />--%>

                <SeriesItems>
                    <telerik:PieSeriesItem Name="Slice1" Y="60"></telerik:PieSeriesItem>
                    <telerik:PieSeriesItem Name="Slice2" Y="40"></telerik:PieSeriesItem>
                </SeriesItems>
            </telerik:PieSeries>
        </Series>
    </PlotArea>
</telerik:RadHtmlChart>