Detect if an element is visible (without using jQuery)

Question

I\'m trying to detect if an html element I gave an id is visible or not without using jquery.

The context:

In the forgotten user password page,

Answer 1

On general_Submit.Label button click call a function Callfun() and then disabled button

" />

    function Callfun()
    {
     document.getElementById("general_Submit.Label").disabled = true;
    }