Looks like there's not really a standard way browsers are handling this, everyone does their own thing based on their own shortcuts and their own view on whether they should be overridable or not.
Example:
This code from a related stackoverflow question: https://stackoverflow.com/a/10919725
In Orion:
cmd+L/M/T/N/Q/F/D/... is overridden by the website, the browser no longer works as expected for these shortcuts.
In Safari:
cmd+L/M/T/N/Q/... is not overridden, the browser still works as expected for these shortcuts.
cmd+F/D/... is overridden by the website, the browser no longer works as expected for these shortcuts.
In Chrome:
cmd+T/N/Q/... is not overridden, the browser works as expected for these shortcuts.
cmd+L/M/... is overridden by the website, the browser no longer works as expected for these shortcuts.