Which event should be cancelled to stop a contenteditable keyboard shortcut?

I am trying to build around contenteditable by first disabling it’s text styling behavior from keyboard shortcuts, such as ctrl+b or ctrl+i.

I am using event.preventDefault() when these shortcuts are detected on keyup and keydown events — however sometimes rarely, I am noticing that contenteditable still ignores event.preventDefault() and the keyboard shortcut and styles/unstyles the selected text with it’s own way.

Is there any way I can fully disable these keyboard shortcuts other than cancelling keyup and keydown?