Edit in JSFiddle

if (!('reversed' in document.createElement('ol'))) {

    (function () {

        // run the code on each ordered list with a 'reversed' attribute.
        var myLists = $('ol[reversed]'),
            currCount = null,
            currChildren = null,
            val;

        myLists.each(function () {

            // check the existence of the start attribute
            if ($(this).attr('start')) {

                // if it exists, convert it to an integer; also ensures even decimal values work
                currCount = parseInt($(this).attr('start'), 10);

                // If it wasn't a number, it will return 'NaN'; if so, use the number of list items instead
                if (isNaN(currCount)) {
                    currCount = $(this).children().length;
                }

            // do this if the start attribute is not present
            // the first number is derived from the number of list items
            } else {
                currCount = $(this).children().length;
            }

            // grab all the child list items
            currChildren = $(this).children();

            // go through each list item, adding the 'value' attribute plus currCount number
            // then subract one from currCount so we're ready for the next one
            currChildren.each(function () {

                val = $(this).attr('value');

                // here we check if a value attribute is set in the html
                // and we change the count accordingly
                if (val && !isNaN(val)) {
                    $(this).attr('value', val);
                    currCount = val;
                }

                $(this).attr('value', currCount);
                currCount = currCount - 1;

            });

        });

    }());

}

<h1>PolyFill for HTML5's Reverse Ordered Lists</h1>

<p>See: <a href="http://www.impressivewebs.com/reverse-ordered-lists-html5">HTML5 Reverse Ordered Lists</a> and: <a href="http://dev.w3.org/html5/spec/Overview.html#the-ol-element">The <code>ol</code> Element</a></p>

<p>(The list below is just a normal reversed list)</p>

<ol reversed>
    <li>List item one
    <li>List item two
    <li>List item three
</ol>

<p>(The list below has two nested lists; one reversed, one not)</p>
 
<ol reversed>
    <li>List item one
    <li>List item two

        <ol reversed>
               <li>Nested one
               <li>Nested two
               <li>Nested three
               <li>Nested four
        </ol>

    <li>List item three
    <li>List item four
    <li>List item five
        
        <ol>
               <li>Nested one
               <li>Nested two
               <li>Nested three
               <li>Nested four
        </ol>

    <li>List item six
    <li>List item seven
</ol>

<p>(No "reversed" on the list below)</p>

<ol>
    <li>List item one
    <li>List item two
    <li>List item three
    <li>List item four
</ol>

<p>(The list below has a "start" attribute)</p>
        
<ol reversed start=435>
    <li>List item one
    <li>List item two
    <li>List item three
    <li>List item four
    <li>List item five
    <li>List item six
</ol>
        
<p>(The reversed list below has <code>value=55</code> set on a single <code>li</code> element)</p>
        
<ol reversed>
    <li>List item one
    <li>List item two
    <li value=55>List item three
    <li>List item four
    <li>List item five
    <li>List item six
</ol>

<p>(The list below uses XHTML syntax for the "reversed" attribute)</p>

<ol reversed="reversed">
    <li>List item one
    <li>List item two
    <li>List item three
    <li>List item four
    <li>List item five
    <li>List item six
</ol>

body {
     color: #555;
     font-family: Georgia, "Times New Roman", serif;
     line-height: 1.5;
}

h1 {
     font-family: Arial, sans-serif;
     font-size: 1.5em;
     color: #555;
}