From the Wiki:
HTML Window Design
You can define the size and position of the window using any of the following attributes to the HTML body element:
HTML Body Attribute Definition/Use data-kmwindow May be either: a size (width, height); a rectangle (left, top, width, height) data-kmwidth width data-kmheight height data-kmleft position of left side data-kmtop position of top data-kmwindowid an ID that can be used with the Execute a JavaScript in Custom Prompt action For example:
<body data-kmwindow="SCREEN(Main,Left,20%),SCREEN(Main,Top,20%),420,560">
So how about trying
<body data-kmleft="MOUSEX()" data-kmtop="MOUSEY()">
Add a few pixels to MOUSEY() if you want to account for the title bar and remember that the edge of your screen is a limit -- the window will spawn as close to your pointer as it can while remaining entirely on screen.