The code 
works to create a <br>
in a tooltip in Firefox, like this:
<div title:"Line1
Line2">hover me</div>
But on Chrome, it just displays as one line.
Am I doing something wrong? Is there another similar piece of code that works on both browsers?
Apparently, the "universally accepted" line break character is
<div title="Line1 Line2">Hover and see two lines</div>
Ref: Web Tooltip "universally accepted" linebreak character?