Opinion: in HTML, Possible Duplicate IDs or Non-Standard Attributes?

<div class="thing" id="5">
<script> myThing = select(".thing#5") </script>

<div data-thing-id="5">
<script> myThing = select("[data-thing-id=5]") </script>

<div class="thing" id="5">

<div class="thing" id="myapp-thing-5"/>

// Get thing on the page for a particular ID
var myThing = select("#myapp-thing-5");

// Get ID for the first thing on the page
var thing_id = /myapp-thing-(d+)/.exec ($('.thing')[0].id)[1];

var whoKnows = document.getElementById('duplicateId');

<div class="friend04"/>
<div class="featuredFriend04" />

<div class="friend friend04" />
<div class="featuredFriend friend04" />

<div class="friend objectId04" />
<div class="groupMember objectId04" />

<div class="friend objectId04" />
<div class="friend objectId04" id="featured" />

<div class="thing myapp-thing-5" />
<div class="thing myapp-thing-668" />
<div class="thing myapp-thing-5" />

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <script>
            window.addEventListener("DOMContentLoaded", function() {
                var thing5 = document.evaluate('//*[@data-thing="5"]',
                document, null, XPathResult.FIRST_ORDERED_NODE_TYPE ,null);
                alert(thing5.singleNodeValue.textContent);
            }, false);
        </script>
    </head>
    <body>
        <div data-thing="5">test</div>
    </body>
</html>