<script>
    function OnClientLoad(sender, args) {
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }

    function OnClientSelectedIndexChanged(sender, args) {
        $telerik.$(sender.get_dropDownElement()).find(".rcbSelected").removeClass("rcbSelected");
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }
</script>

<style>
    div.RadComboBoxDropDown .rcbSelected {
        color: red;
        background-color: lightgreen;
    }
</style>
<telerik:RadComboBox ID="RadComboBox1" runat="server" RenderMode="Lightweight" OnClientSelectedIndexChanged="OnClientSelectedIndexChanged" OnClientLoad="OnClientLoad" MaxHeight="150">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" />
        <telerik:RadComboBoxItem Text="Item 2" />
        <telerik:RadComboBoxItem Text="Item 3" Selected="true" />
        <telerik:RadComboBoxItem Text="Item 4" />
    </Items>
</telerik:RadComboBox>

This element has a mismatch between the value of its readonly and aria-readonly attributes

<telerik:RadComboBox ID="RadComboBox1" runat="server" EmptyMessage="Empty Message" EnableAriaSupport="true">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" />
        <telerik:RadComboBoxItem Text="Item 2" />
        <telerik:RadComboBoxItem Text="Item 3" />
        <telerik:RadComboBoxItem Text="Item 4" />
    </Items>
</telerik:RadComboBox>

<telerik:RadComboBox
    ID="ddlOldValues"
    runat="server"
    Filter="Contains"
    AllowCustomText="true"
    ShowMoreResultsBox="true"
    EnableLoadOnDemand="true"
    ItemsPerRequest="10"
    EnableVirtualScrolling="true"
    DataTextField="USERFULLNAME"
    DataValueField="USERFULLNAME">
    <WebServiceSettings Method="GetDynamicUsersUS" Path="WebService.asmx" />
</telerik:RadComboBox>

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class WebService : System.Web.Services.WebService
{
    [WebMethod(EnableSession = true)]
    public RadComboBoxData GetDynamicUsersUS(RadComboBoxContext context)
    {
        RadComboBoxData comboData = new RadComboBoxData();

        try
        {
            DataTable data = GetUsersData();
            string filter = "USERFULLNAME LIKE '%" + context.Text + "%'";
            data.DefaultView.RowFilter = filter;
            DataTable dtData = data.DefaultView.ToTable();

            if (dtData == null || dtData.Rows.Count == 0)
            {
                dtData = data;
            }

            // Exact same GetComboData logic as frmBRMultipleUpdate.aspx.vb
            int itemsPerRequest = 10;
            int itemOffset = context.NumberOfItems;
            int endOffset = itemOffset + itemsPerRequest;

            if (endOffset > dtData.Rows.Count)
            {
                endOffset = dtData.Rows.Count;
            }

            // When data is exhausted, set EndOfItems to true
            if (itemOffset >= dtData.Rows.Count)
            {
                comboData.EndOfItems = true;
                comboData.Items = new RadComboBoxItemData[0];
                return comboData;
            }

            comboData.EndOfItems = (endOffset >= dtData.Rows.Count);

            var result = new List<RadComboBoxItemData>();
            for (int i = itemOffset; i < endOffset; i++)
            {
                var item = new RadComboBoxItemData();
                item.Text = dtData.Rows[i]["USERFULLNAME"].ToString();
                item.Value = dtData.Rows[i]["USERFULLNAME"].ToString();
                result.Add(item);
            }

            comboData.Items = result.ToArray();
            comboData.Message = string.Format("Items <b>{0}</b>-<b>{1}</b> out of <b>{2}</b>",
                itemOffset + 1, endOffset, dtData.Rows.Count);
        }
        catch (Exception ex)
        {
            comboData.Message = "No matches";
        }
        return comboData;
    }

    private static DataTable GetUsersData()
    {
        DataTable dt = new DataTable();

        dt.Columns.Add(new DataColumn("USERFULLNAME", typeof(string)));
        dt.Columns.Add(new DataColumn("EMAIL", typeof(string)));

        for (int i = 0; i < 200; i++)
        {
            int index = i + 1;

            DataRow row = dt.NewRow();

            row["USERFULLNAME"] = "User" + index;
            row["EMAIL"] = "user" + index + "@example.com";

            dt.Rows.Add(row);
        }

        return dt;
    }
}

<telerik:RadComboBox
    Height="300px"
    ID="ddlOldValues"
    ...