What are cross-browser and cross-OS safe keyboard shortcuts usable for web application?

Question

I am developing a quite large web application, and it is probably a good idea to use hotkeys for some common tasks. However, I discovered that finding safe key combinations is a

Answer 1

Facebook uses the following ones that need different main keys to press depending on the used browser:

https://www.facebook.com/help/156151771119453?helpref=faq_content

Key Combinations for Different Browsers

Find the right combination of keys for your browser in the list below, and replace # with the access key number listed under access keys below.

Internet Explorer for PC: Alt + #, then Enter
Firefox for PC: Shift + Alt + #
Safari for Mac: Ctrl + Opt + #
Firefox for Mac: Ctrl + Opt + #
Chrome for Mac: Ctrl + Opt + #
Chrome for PC: Alt + #

Access keys
0 - Help
1 - Home
2 - Timeline
3 - Friends
4 - Inbox
5 – Notifications
6 – Settings
7 - Activity Log
8 - About
9 - Terms

However, Facebook supports these shortcuts in all browsers:

Web Messenger Keyboard Shortcuts
Ctrl + G - Search conversations
Ctrl + Q - Show/hide keyboard shortcuts
Ctrl + Delete - Archive/unarchive conversation
Ctrl + J - Mark as spam
Ctrl + M - Start a new message
Ctrl + I - Go to Inbox
Ctrl + U - Go to Other

Conclusion: Some shortcuts might work in all browsers, but most of them need additional explanation.