Event handlers are a vital tool for web developers, because they allow us to build a script that will run only if and when a specific event occurs. Introduced in HTML 4.0, they add a dynamic element to a website, allowing programs to run in response to your visitors' actions.

Event Handler	Trigger
onabort	Visitor cancels a loading image
onblur	An element loses focus (e.g. visitor clicks out of a form field)
onchange	Visitor changes the content of a field
onclick	Visitor clicks an object with the mouse cursor
ondblclick	Visitor double-clicks an object
onerror	An error occurs when loading a document or an image
onfocus	An element gets focus (e.g. visitor clicks on or tabs into a form field)
onkeydown	Visitor presses a key
onkeypress	Visitor presses or holds down a key
onkeyup	Visitor releases a key
onload	A page or an image finishes loading
onmousedown	Visitor presses the mouse button
onmousemove	Visitor moves the mouse
onmouseout	Visitor moves the mouse cursor off an element
onmouseover	Visitor moves the mouse cursor onto an element
onmouseup	Visitor releases the mouse button
onreset	Visitor clicks the Reset button of a form
onresize	Visitor resizes the frame or window
onselect	Visitor selects a portion of text
onsubmit	Visitor clicks the Submit button of a form
onunload	Visitor exits the page

You can insert event handlers directly into HTML tags, like this...

<input type="button" value="Click me!" onclick="alert('You clicked the button!')">

... which will produce the following effect: