UJML 2.1 provides performance improvements and some new language features: 

Reusable Components, supporting

• Loading byte code for new components.
• Creating and managing instances of components.
• Calling functions on an instance of a component.
• Receiving events from a component.
• Painting an instance of a component.

Resizable Arrays

• Declarations are the same as in previous versions of UJML except that when the dimensions are set to zeros the array can be resized.
• You can use the _getArrayLength() and _resizeArray() functions to work with resizable arrays.

New and changed elements for UJML version 2.1:

• New break and continue keywords
• Changes to the <transition> tag

New elements for UJML version 2.0: 

Tags:

• <component>
• <display-instance>
• <instance>
• <interface>
• <interfaces>
• <name>

Functions

• _castInterface()
• _createInstance()
• _getArrayLength()
• _getSelf()
• _instanceOf()
• _isSameInstance()
• _resizeArray()

Obsolete elements as of UJML version 2.0: 

Tags

• <display-handle>
• <extension>
• <extensions>
• <handle>
• <use>
• <uses>

Functions

• _isExtensionAvailable()