Allowed HTML Elements
You can now control what HTML markup elements (tags) are allowed to be entered into your discussion messages.
There is a list of builtin markup elements that can be toggled between allowed and not allowed. The list of builtin elements is from XHTML Modularization 1.1 - Second Edition.
You can also add your own custom elements.
See How To Edit Allowed HTML Elements.
When a non-allowed element is entered it is escaped. The escaping happens after the markup language for your message is rendered.