HTML and CSS Reference
Figure I.11. Although the mouse pointer has moved to the right, Wilson has not followed because the pointer
movement occurred outside of the bounds of the element.
The Mouse Event Capture API is not yet part of any standard, but that's more because
no one has decided where to put it than that it's not useful or won't be standardized. The
HTML5 and W3C recommendation requirement of “two compatible implementations” has
already been met. It's possible it will appear in the DOM Level 3 specification.