HTML 5 data-* attributes, how to use them and why

It is always tempting to add custom attributes in HTML so that you can use the data stored there to do X. But if you do that, there is no way of knowing if your HTML attribute will not be overridden in the future and used for something else. Additionally, you will not be writing valid HTML markup that can pass HTML 5 validator, and with that, you can create some nasty side effects. That is why there is a spec in HTML 5 called custom data attributes that enable several useful features.

You may go around and read the specs, but the basic idea is straightforward, you can add any attribute that starts with "data-" That attribute will be treated as non-visible data for that attribute. By non-visible, it is not something that gets rendered to the client, so it does not affect the layout or style of the page, but it is there in the HTML, so in no way is this private.
So let's get right into it; the following snippet is valid HTML5 markup.

  <div id="awesome" 
     data-hash="3e4ae6c4e30e50e4fdfc7bf439a09974">Some awesome data</div>

Great, so now how do you read the data from there? You can go through the elements and get the attributes you need or to go and use jQuery if it is already there. Using jQuery's .data() API, you can work with the "data-*" elements. One method is .data(obj) added in jQuery1.4.3 that basically retrieves data for the selected element.
For example, if you want to get the value of the data-hash attribute with the following snippet:

 var dataHashValue = jQuery("#awesome").data('hash');

You can also use a JSON syntax in the data fields, for example, if you have the following HTML:

<div id="awesome-json" data-awesome='{"game":"on"}'></div> 

Accessing data from js can be done directly just by adding the JSON key to the result:

  var gameStatus= jQuery("#awesome-json").data('awesome').game;

You can also set the data using .data(key, value) directly from JS. One important thing to remember is that the "data-*" attributes should be linked to the element somehow. They should add additional info about that element or the data it contains and not act just as a storage for everything. 

You can go around and play with it in the following jsfidde.

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