document.execCommand('copy') support across browsers, from most permissive to most restrictive (updated 2017-02-13)
Chrome, Chrome for Android allows sync copy and async copy (up to 1 second)
Issue 424968 - chromium - Cannot access to clipboard access via Javascript - Monorail
Opera allows sync copy and async copy (up to 1 second)
Safari for OS X allows sync copy and async copy (as of Safari 10; up to 1 second)
Internet Explorer 11 allows sync copy and async copy (indefinite delay?) with user prompt
Firefox, Firefox for Android allows sync copy but not async copy
1012662 – Allow document.execCommand("cut"/"copy") to be used within the context of user generated events 1151429 – Allow JS to copy text to the clipboard by triggering document.execCommand('copy') or document.execCommand('cut') in user-initiated thread 1185052 – Consider extending IsHandlingUserInput to also return true in event callbacks triggered by a user event 1197451 – Support clipboard access in open extension API 1266654 – document.execCommand('copy') is unavailable for context menu command
Edge allows sync copy but not async copy
Allow async call to document.execCommand('copy') from handler of user-initiated event
iOS (all browsers) does not allow sync copy or async copy; no Flash support, so no opportunity for ZeroClipboard workaround
Copy buffer style: Async delay:
Tried to copy: