Your target selectors may use this pseudo-selector
to replace the layer's topmost swappable element.
The topmost swappable element is the first child of the layer's container element.
For the root layer it is the <body> element. For an overlay
it is the target with which the overlay was opened with.
In canonical usage the topmost swappable element is often a main element.
Example
The following will replace the <body> element in the root layer,
and the topmost swappable element in an overlay:
up.render(':layer', { url: '/page2' })
