Design time check of markup extension arguments in WPF designer

I've written a Markup extension for WPF that allows me to do

<!-- Generic Styles -->
<Style x:Key="bold" TargetType="Label">
    <Setter Property="FontWeight" Value="ExtraBold" />
</Style>

<Style x:Key="italic" TargetType="Label">
    <Setter Property="FontStyle" Value="Italic" />
</Style>

<Style x:Key="gridHeader" TargetType="Label" 
    BasedOn="{WPF:CombiStyle bold italic }" >

It is a very usefull extension and it works great at runtime. However at design time I can't see the styles applied or that if I mistype bold and italic they might not be found as StaticResources.

Any hacks I can do to get this working?

The source code for the extension is

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Markup;

namespace MarkupExtensions
{
    [MarkupExtensionReturnType(typeof(Style))]
    public class CombiStyleExtension : MarkupExtension
    {

        private string[] MergeStyleProviders { get; set; }

        public CombiStyleExtension(string s0)
        { 
            MergeStyleProviders = s0.Split(new[]{' '});
        }

        public override object ProvideValue(IServiceProvider
                                            serviceProvider)
        {
            return MergeStyleProviders
                .Select(x => StringToStyle(serviceProvider, x))
                .Aggregate(new Style(), RecursivelyMergeStyles);
        }

        private static Style StringToStyle(IServiceProvider serviceProvider, string x)
        {
            var style = new StaticResourceExtension() { ResourceKey = x }.ProvideValue(serviceProvider) as Style;
            if (style==null)
            {
                throw new ArgumentException("Argument could not be converted to a style");
            }
            return style;
        }

        private static S