HTML5: Adding context menu entries for selected elements?

I've looked up how to realize this but so far only found methods how ta create a custom right-click menu. That's not what I'm intending, I'm searching method for how to reuse a built-in context menu with a standard look for adding my own entries. On old legacy Firefox browsers this was possible by inserting MENU / MENUITEM tags anywhere into HTML content. This unfortunately was disabled in modern browsers, so the only way seems to me to create a completely new custom menu. I'm curious if touching the built-in menu is still somehow possible, even by a user's hack to undocumented functions or similar.