app:

<aura:application description="LockerTestApp">
    <lightning:button onclick="{!c.buttonClicked}" label="A button"/>
    <button onclick="{!c.buttonClicked}">Another button</button>
</aura:application>

controller:

({
    buttonClicked : function(component, event, helper) {
        helper.printButtons(component);
    }
})

helper:

({
    printButtons : function(component) {
        var buttons = document.getElementsByTagName('button');
        console.log(buttons.length);
        for(var i=0; i < buttons.length; i++) {
            console.log('button number ' + i + ' ' + buttons[i].textContent);
        }
    }
})

renderer:
({
    afterRender: function (component, helper) {
        this.superAfterRender();
        console.log('after render');
        helper.printButtons(component);
    }
})