I was a little quick..

[Acorn]

Anything that is not on the MouseOff layer will not be visible when the page loads.

Not quite Gary.

When a new web document is first created, Xara hides the MouseOver layer. Xara treats the MouseOver layer as a special layer (c.f., MouseDown & Page and Pasteboard backgrounds). The MouseOver layer is always hidden on rendering.

Anything that is added to any new top layers will appear if the layer is visible. If hidden, and the layer has no incoming links, the shapes are not published.
Only when you add a button, NavBar or other shape that links to a layer is when that layer stays hidden on the web page until activated: click, rollover or other event. All the layer shapes are also published.
You can create a link (popup: close "layerName") that never opens a layer, yet the layer shapes are published - hiding in plain sight.

Acorn