Hi Stefan,
I just checked using the Explorer app samples (https://sapui5.hana.ondemand.com/explored.html#/entity/sap.viz.ui5.controls.VizFrame/samples) and found that there is no library.css resource.
There is a library.js resource but the HTTP call to it uses the HEAD verb - which is really just the runtime checking if the resource has been updated since the latest download.
Cheers
Graham Robbo