HTML5 add new data attribute

The data attribute in HTML5  must be prefixed with 'data-'. It should not contain any uppercase letters.

To access the data attribute in javaScript

<div id='mypages' data-spacing='10' data-line-width='10.0 cm'></div>
<script>
// 'Getting' data-attributes using dataset 
var mypages = document.getElementById('mypages');
var spacing = mypages.dataset.spacing; // 10;
// 'Setting' data-attributes using dataset
var linewidth = plant.dataset.lineWidth; // 'line-width' -> 'lineWidth'
</script>

Reference:

http://html5doctor.com/html5-custom-data-attributes/