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In generic terms event handlers can be described as those commands which are optional and are required only when a change occurs either with the host or the service. All the interactions between the user and any web page always get tracked by web browser through events. Therefore, even the smallest activity that is initiated by the user is an event; which can include anything from clicking on a button or loading or reloading a web page or placing the mouse on the image or removing the cursor from the image to hyperlink and many more. Event handlers are nothing but a simple solution provided by Java Script for responding to all the user actions. Event Handlers are always accompanied by HTML code and can be triggered either by a user or a browser.

Events can be separated into two major classes, user related events and documents related events. User related events are nothing extraordinary, they are the standard keyboard and the mouse events that occur when a user handles any of the two and are generic in all the HTML elements and can be classified as mouse and the keyboard events. These would mainly include onKeyDown, onKeyPress, onkeyUp, onDblClick, onMouseUp, onMouseDown and onClick attributes. The documents related events are those special events that occur either when the browser is displaying and managing the HTML documents or are mainly related to the handling of different types of states and actions of HTML form controls. These would majorly include onSubmit, onReset, onChange and onSelect attributes.

The event handlers can also be classified based on their dependencies on the computer devices; event handlers which are device dependent, as the name describes are those event handlers directly dependent on the device such as mouse or keyboard and the others are those ones which are device independent, it does not mean that they are not dependent on any device, they definitely are device dependent but not directly rather through some other means.

The device dependent event handlers in 4.01 version of HTML include ondblclick, onMouseover and onMouseout and are mainly used in the situations where only the presentation of any particular element needs to be changed; while the device independent event handlers include onSelect, onBlur and onFocus primarily find their use in those situations when the even handler is supposed to handle or perform much more than just changing the presentation of any particular element. Whenever using the device dependent event handlers, always remember to test them thoroughly in order to ensure that no additional keyboard event would be required later.

Through the related ‘on’ event attributes, HTML is capable of supporting a rich set of event handlers. Either a quoted string which comprises of one or more script statements or the name of a function which may be described elsewhere in the document can be used to depict the value of the event handler.