Here's what I found about the topic in general:
Buffering INSIDE a polygon with ArcGIS (possible): ArcGIS: inside buffer with attributes?
Buffering a polygon on BOTH sides with JSTS (possible): Creating a Polyline Buffer in OpenLayers
But I need to buffer a polygon on just the inside. Either a "real" buffer with the output geometry, or a "fake" buffer with just a line thickness and style. However line thickness in OL goes on both sides of the line.
End goal is to get something like this:
Side-question - Does any know if it's possible to do the country outline (like above) via Google Maps API?! Have not been able to find any reference. So I'm trying to duplicate with OpenLayers and a geojson outline of the world.