How to prevent Command-I/Command-Shift-I from opening Mac Mail when in browser?
You can change the shortcuts from System Preferences:



If you want to disable the shortcuts, edit the NSUserKeyEquivalents dictionaries directly:

defaults write com.google.Chrome NSUserKeyEquivalents -dict-add 'Email Page Location' '\0'
defaults write com.google.Chrome.canary NSUserKeyEquivalents -dict-add 'Email Page Location' '\0'
defaults write com.apple.Safari NSUserKeyEquivalents -dict-add 'Email Link to This Page' '\0' 'Email This Page' '\0'


Then quit and reopen Chrome and Safari to apply the changes.