The ECMAScript standard defines how an implementation should react to a language construct. Other non-ECMA-compliant implementations may behave in the same way most of the time and may deviate from the standard at others.

When the implementation conforms to the standard, its behavior is predictable according to the definitions of the standard. When an implementation is not conformant, it may behave according to one of the following abnormal behavior models:

• Unspecified behavior

• Undefined behavior

• Implementation-defined behavior

• Locale-specific behavior

Another meaning for the word behavior in the context of JavaScript is the way that MSIE supports the addition of JavaScript functionality to style definitions. This is covered under the descriptions of the addBehavior() and removeBehavior() methods that belong to the Element object.