JavaScript 'visibility' or 'display' - works in one copy not in identical copy

Dear Colleagues:

                            I have some javascript which is based on using the 'display' or the 'visibility' properties - either one is fine.  

The line of code that needs to either display or be hidden reads:

<span id="ifyes" style="display:none">  Blah, blah, blah...</span>

...this line using either 'display' or 'visibility' works fine - with the style in place it vanishes.

However, when one adds this javascript

...it works in some scripts and not in others.

<script type="text/javascript">
    function yesnoCheck() {
        if (document.getElementById('yesCheck').checked) {
            document.getElementById('ifYes').style.display = 'block';
        } else document.getElementById('ifYes').style.display = 'none';

    }

...that works and can be seen working as a jsfiddle at:

http://jsfiddle.net/QAaHP/50/

...I finally hand typed the identical code into another jsfiddle window...

http://jsfiddle.net/Qqj5R/52/

...it DOES NOT work, and yet it appears identical.  Running in a different browser and/or on a different O/S makes no difference (checked just in case...).

I need this functionality working in my own production code and I can not get it to work - can anyone tell me why the one script works and the other does not?  If I know why in these examples I can fix my own similar code.