Toggle display:none style with JavaScript

Give your ul an id,

<ul id='yourUlId' class="subforums" style="display: none; overflow-x: visible; overflow-y: visible; ">

then do

var yourUl = document.getElementById("yourUlId");
yourUl.style.display = yourUl.style.display === 'none' ? '' : 'none';

IF you're using jQuery, this becomes:

var $yourUl = $("#yourUlId"); 
$yourUl.css("display", $yourUl.css("display") === 'none' ? '' : 'none');

Finally, you specifically said that you wanted to manipulate this css property, and not simply show or hide the underlying element. Nonetheless I'll mention that with jQuery

$("#yourUlId").toggle();

will alternate between showing or hiding this element.


Give the UL an ID and use the getElementById function:

<html>
<body>
    <script>
    function toggledisplay(elementID)
    {
        (function(style) {
            style.display = style.display === 'none' ? '' : 'none';
        })(document.getElementById(elementID).style);
    }
    </script>

<a href="#" title="Show Tags" onClick="toggledisplay('changethis');">Show All Tags</a>
<ul class="subforums" id="changethis" style="overflow-x: visible; overflow-y: visible; ">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
</ul>

</body>
</html>

Others have answered your question perfectly, but I just thought I would throw out another way. It's always a good idea to separate HTML markup, CSS styling, and javascript code when possible. The cleanest way to hide something, with that in mind, is using a class. It allows the definition of "hide" to be defined in the CSS where it belongs. Using this method, you could later decide you want the ul to hide by scrolling up or fading away using CSS transition, all without changing your HTML or code. This is longer, but I feel it's a better overall solution.

Demo: http://jsfiddle.net/ThinkingStiff/RkQCF/

HTML:

<a id="showTags" href="#" title="Show Tags">Show All Tags</a>
<ul id="subforms" class="subforums hide"><li>one</li><li>two</li><li>three</li></ul>

CSS:

#subforms {
    overflow-x: visible; 
    overflow-y: visible;
}

.hide {
    display: none; 
}

Script:

document.getElementById( 'showTags' ).addEventListener( 'click', function () {
    document.getElementById( 'subforms' ).toggleClass( 'hide' );
}, false );

Element.prototype.toggleClass = function ( className ) {
    if( this.className.split( ' ' ).indexOf( className ) == -1 ) {
        this.className = ( this.className + ' ' + className ).trim();
    } else {
        this.className = this.className.replace( new RegExp( '(\\s|^)' + className + '(\\s|$)' ), ' ' ).trim();
    };
};