A new feature introduced in HTML 5 is the addition of custom data attributes.
A custom data attribute starts with data- and would be named based on your requirement.
<div class = "example" data-sports = "cricket" data-level = "complex"> ... </div>
The above will be perfectly valid HTML5 with two custom attributes called data-subject and data-level. You would be able to get the values of these attributes using JavaScript APIs or CSS in a similar way as you get for standard attributes.