Custom attributes – Yea or nay?

Custom attributes – Yea or nay?

Recently I have been reading more and more about people using custom attributes in their HTML tags, mainly for the purpose of embedding some extra bits of data for use in javascript code.
I was hoping to gather some feedback on whether or not using custom attributes is a good practice, and also what some alternatives are.
It seems like it can really simplify both server side and client side code, but it also isn’t W3C compliant.
Should we be making use of custom HTML attributes in our web apps? Why or why not?
For those who think custom attributes are a good thing: what are some things to keep in mind when using them?
For those who think custom attributes are bad thing: what alternatives do you use to accomplish something similar?
Update: I’m mostly interested in the reasoning behind the various methods, as well as points as to why one method is better than another. I think we can all come up with 4-5 different ways to accomplish the same thing. (hidden elements, inline scripts, extra classes, parsing info from ids, etc).
Update 2: It seems that the HTML 5 data- attribute feature has a lot of support here (and I tend to agree, it looks like a solid option). So far I haven’t seen much in the way of rebuttals for this suggestion. Are there any issues/pitfalls to worry about using this approach? Or is it simply a ‘harmless’ invalidation of the current W3C specs?

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Solution 1:

HTML 5 explicitly allows custom attributes that begin with data. So, for example, <p data-date-changed="Jan 24 5:23 p.m.">Hello</p> is valid. Since it’s officially supported by a standard, I think this is the best option for custom attributes. And it doesn’t require you to overload other attributes with hacks, so your HTML can stay semantic.


Solution 2:

Here’s a technique I’ve been using recently:

<div id="someelement">

    <!-- {
        someRandomData: {a:1,b:2},
        someString: "Foo"
    } -->

    <div>... other regular content...</div>

The comment-object ties to the parent element (i.e. #someelement).

Here’s the parser:

To get the data for any particular element simply call parseData with a reference to that element passed as the only argument:

var myElem = document.getElementById('someelement');

var data = parseData( myElem );

data.someRandomData.a; // <= Access the object staight away

It can be more succinct than that:

<li id="foo">
    ... content ...

Access it:

parseData( document.getElementById('foo') ).specialID; // <= 245

The only disadvantage of using this is that it cannot be used with self-closing elements (e.g. <img/>), since the comments must be within the element to be considered as that element’s data.


Notable benefits of this technique:

  • Easy to implement
  • Does not invalidate HTML/XHTML
  • Easy to use/understand (basic JSON notation)
  • Unobtrusive and semantically cleaner than most alternatives

Here’s the parser code (copied from the hyperlink above, in case it ever becomes unavailable on

var parseData = (function(){

    var getAllComments = function(context) {

            var ret = [],
                node = context.firstChild;

            if (!node) { return ret; }

            do {
                if (node.nodeType === 8) {
                    ret[ret.length] = node;
                if (node.nodeType === 1) {
                    ret = ret.concat( getAllComments(node) );
            } while( node = node.nextSibling );

            return ret;

        cache = [0],
        expando = 'data' + +new Date(),
        data = function(node) {

            var cacheIndex = node[expando],
                nextCacheIndex = cache.length;

            if(!cacheIndex) {
                cacheIndex = node[expando] = nextCacheIndex;
                cache[cacheIndex] = {};

            return cache[cacheIndex];


    return function(context) {

        context = context || document.documentElement;

        if ( data(context) && data(context).commentJSON ) {
            return data(context).commentJSON;

        var comments = getAllComments(context),
            len = comments.length,
            comment, cData;

        while (len--) {
            comment = comments[len];
            cData =\n|\r\n/g, '');
            if ( /^\s*?\{.+\}\s*?$/.test(cData) ) {
                try {
                    data(comment.parentNode).commentJSON =
                        (new Function('return ' + cData + ';'))();
                } catch(e) {}

        return data(context).commentJSON || true;



Solution 3:

You can create any attribute if you specify a schema for your page.

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For example:


<html xmlns="" xmlns:addthis="">
<a addthis:title="" addthis:url="" ...>

Facebook (even tags)

<html xmlns:og="" xmlns:fb="">
<fb:like href="" width="450" height="80"/>

Solution 4:

The easiest way to avoid use of custom attributes is to use existing attributes.

use meaningful, relevant class names.
For example, do something like: type='book' and type='cd',
to represent books and cds. Classes are much better for representing what something IS.

e.g. class='book'

I have used custom attributes in the past, but honestly, there really isn’t a need to for them if you make use of existing attributes in a semantically meaningful way.

To give a more concrete example, let’s say you have a site giving links to different kinds of stores. You could use the following:

<a href='wherever.html' id='bookstore12' class='book store'>Molly's books</a>
<a href='whereverelse.html' id='cdstore3' class='cd store'>James' Music</a>

css styling could use classes like:

.store { } { } { }

In the above example we see that both are links to stores (as opposed to the other unrelated links on the site) and one is a cd store, and the other is a book store.

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Solution 5:

Embed the data in the dom and use metadata for jQuery.

All the good plug-ins support the metadata plugin(allowing per tag options).

It also allows infinitely complex data/data structures, as well as key-value pairs.

<li class="someclass {'some': 'random,'json':'data'} anotherclass">...</li>


<li class="someclass" data="{'some':'random', 'json': 'data'}">...</li>


<li class="someclass"><script type="data">{"some":"random","json":"data"}</script> ...</li>

Then get the data like so:

var data = $('li.someclass').metadata();
if ( data.some && data.some == 'random' )
alert('It Worked!');

Solution 6:

I see no problem in using existing XHTML features without breaking anything or extending your namespace. Let’s take a look at a small example:

<div id="some_content">

How to add additional information to some_content without additional attributes? What about adding another tag like the following?

<div id="some_content">
 <div id="some_content_extended" class="hidden"><p>Some alternative content.</p></div>

It keeps the relation via a well defined id/extension “_extended” of your choice and by its position in the hierarchy. I often use this approach together with jQuery and without actually using Ajax like techniques.