Question

How to “implement” back and forward buttons in an iframe with JS?

Answer 1

Use the window.history object.

// For the current window
window.history.back();     
window.history.forward();

// For an iframe's window
iframe.contentWindow.history.back(); 
iframe.contentWindow.history.forward();

or

iframe.contentWindow.history.go(-1); // back
iframe.contentWindow.history.go(1);  // forward

https://developer.mozilla.org/en/dom/window.history