The onmouseover attribute is one of the most commonly used event attributes. It captures the moment that a cursor crosses the boundary of an element, moving from outside to inside the element to which the attribute is applied. It differs from the onmousemove attribute, which is used to detect movement within the element’s boundaries. Once the cursor is positioned over the element, the onmouseover event remains active until the cursor is moved beyond the element’s boundaries—an event that the onmouseout attribute would capture.
Note that this event attribute cannot be applied to the following elements:
The example below shows a simple
image swap technique, whereby mousing over the image causes the image to
change to one that reveals a location on a map:
<div> <img src="map.gif" alt="Hover to reveal the location on the map" onmouseover="this.src='map_location_revealed.gif';" onmouseout="this.src='map.gif';"/>Figures for February’s racing. </div>
This attribute has no fixed value. It’s up to the author to decide on the scripting that’s included here, be that a call to one or more defined functions, or a simple alert() statement.
Every browser listed supports this attribute. However, inline event handlers such as this should be avoided. In the same way that inline CSS styles are frowned upon but externally defined CSS styles are considered good practice, inline event handlers should be stripped out and replaced with events attached unobtrusively through the DOM.
The Principles of Beautiful Web Design, 4th Edition
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