Hi Tihomir,
DSCP marking depends really on the browser and OS platform as its internal modules are the ones building up and tagging the Media information.
My understanding is that Chrome DSCP config settings is being there for couple of years (Set 'googdscp' to true) , just bear in mind that depending on the platform and OS you may need to test it.
I found this defect on Mozilla Browser website which still open:
https://bugzilla.mozilla.org/show_bug.cgi?id=1249575
Apart from setting DSCP value at the source which is the recommended options for QOS endpoints, remember you can always rewrite at any entry point in your network and make sure that DSCP value is honored and prioritize end to end.
-Gonzalo