Tabindex is standard html that is used to set the order of which is the next field that chosen to focuses the cursor.
Tabindex for ALL hotspot / layers is a good idea too.
That would allow virtual tours to be used without a mouse and is good for people with disabilities.
I think the good functionality be
<layer tabindex="1" tabindex_ontab="function() " tabindex_onenter="function();"
Bind the onclick to the enterkey as a default or ondown if not found. the ontab allows you to set an action when a new layer/hotspot is selected. Change css text/background, set scale etc.