But for me, no shortcut works in Safari. Using 10.10.2 on two different types of mac, and both will not recognise a shortcut for the DevonThinkPro web extension.
Do you perhaps have the specific keypress sequences you’ve tried defined for other functions in the OS or a different app? “Shift-Cmd-whatever” is a common keypress sequence.
Maybe your shortcuts are colliding with other functionality? Here, for example, the the possible shortcut ⌘⇧i creates a new email message with the link to the shown web page even though I assigned it to Clip to DEVONthink, too. What happens when a shortcut is activated that is connected to two different functions in an app, is up to the app itself (or just sort of random).