Changing classes to internal instead of public in Core project. The abstract classes in the Web project can be used to create providers.

src/Umbraco.Core/Media/IEmbedProvider.cs

@@ -1,6 +1,6 @@
 namespace Umbraco.Core.Media
 {
-    public interface IEmbedProvider
+    internal interface IEmbedProvider
     {
         bool SupportsDimensions { get; }
 

src/Umbraco.Core/Media/IEmbedSettingProvider.cs

@@ -2,7 +2,7 @@
 
 namespace Umbraco.Core.Media
 {
-    public interface IEmbedSettingProvider
+    internal interface IEmbedSettingProvider
     {       
         object GetSetting(XmlNode settingNode);
     }

src/Umbraco.Core/Media/ProviderSetting.cs

@@ -2,7 +2,7 @@
 
 namespace Umbraco.Core.Media
 {
-    public class ProviderSetting : Attribute
+    internal class ProviderSetting : Attribute
     {
     }
 }

src/Umbraco.Web/Media/EmbedProviders/Twitgoo.cs

@@ -2,7 +2,7 @@
 
 namespace Umbraco.Web.Media.EmbedProviders
 {
-    public class Twitgoo: AbstractProvider
+    public class Twitgoo : AbstractProvider
     {
         public override bool SupportsDimensions
         {