Best way to add metadata to HTML elements

Go ahead and use an attribute for this, but use a data- prefix on it. Attributes with the prefix data- are explicitly allowed on all elements as of HTML5. Example:

<a href="#" class="Theme-Button" data-theme="{style: 'win2007', icon: 'left', align:'center', fullWidth: true}"></a>

It works today in all browsers, and because it's now specified behavior, it's future-proofed.

Use jquery's ".data" property. This is very handy and many people don't know about it.

See this link for more information.