It is common that a droppable component doesn't accept all draggable components. For example, an e-mail folder accepts only e-mails and it rejects contacts or others. You could silently ignore non-acceptable components or alert an message, when onDrop is invoked.

To have better visual effect, you could identify each type of draggable components with an identifier, and then assign the identifier to the draggable property.

<listitem draggable="email"/>
...
<listitem draggable="contact"/>

Then, you could specify a list of identifiers to the droppable property to limit what can be dropped. For example, the following image accepts only email and contact.

<image src="/img/send.png" droppable="email, contact" onDrop="send(event.dragged)"/>

To accept any kind of draggable components, you could specify "true" to the droppable property. For example, the following image accepts any kind of draggable components.

<image src="/img/trash.png" droppable="true" onDrop="remove(event.dragged)"/>

On the other hand, if the draggable property is "true", it means the component belongs to anonymous type. Furthermore, only components with the droppable property assigned to "true" could accept it.