We're having some issues with validation of properties on shared blocks in EPiServer 11.

Custom attribute:

[AttributeUsage(AttributeTargets.Property)]
public class ValidYoutubeLink : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        ...
    }
}

Shared block:

[ContentType(GUID = "...")]
public class YoutubeBlock : BlockData
{
    [Display(...)]
    [Required]
    [ValidYoutubeLink]
    public virtual string VideoUrl { get; set; }
}

As an editor, I try to create a new shared block inside For All Sites folder.

In theory, Episerver should validate all the properties before executing EPiServer.DataFactory.Save() 

https://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/7/Validation/Validation/

But in practice, I can enter invalid values, click the Save button and go to next screen!

On the next screen, I'm able to click the Publish button. However, nothing happens when I do that. No validation errors in the edit mode, no meaningful errors in dev. tools console, nothing!