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Buttons for web can be tough, and is often personal preference. Tab could work if you didn't have controls that make sense to navigate with tab. Depending on the game I've seen a number of options. Q, backslash, and backspace (though with web that can be a bad one if they lose focus) are fairly common. Left quote/back tick is also a decent choice being so close to Escape. We will often leave Escape as an option, along with a secondary non-escape key.