onMouseOver

Onmouseover is an HTML attribute that specifies a JavaScript handler to run when the pointer moves onto an element. It is one of several inline event attributes that can respond to user interactions, and it is typically used alongside onmouseout to reset state when the pointer leaves the element.

In the DOM, the mouseover event fires when the pointer enters the element or any of its descendant elements, whereas mouseenter fires only when the pointer enters the element itself. The complementary event is onmouseout, which fires when the pointer leaves the element or its descendants; its counterpart is mouseleave.

Common uses include changing styling, displaying tooltips, or triggering dynamic content such as image rollovers or menus. Example: <div onmouseover="this.style.color='red';" onmouseout="this.style.color='';">Hover me</div>. In JavaScript, equivalent behavior is achieved with element.addEventListener('mouseover', handler) and 'mouseout' listeners.

Best practices emphasize separating structure and behavior. Favor CSS for simple hover effects using the :hover pseudo-class; use JavaScript addEventListener for more complex interactions. Also consider accessibility: rely on focusable elements and onfocus/onblur to support keyboard users, and ensure that content revealed on hover is also accessible via keyboard or click.

Compatibility: Supported in all major browsers. Note that onmouseover may behave differently with touch devices, where hover states are not consistently available. Developers should avoid relying on hover for essential functionality and provide non-hover equivalents.