Debug JavaScript, inspect CSS and the DOM, and view any errors -- Opera Dragonfly makes developing using Opera easier than ever, both on your computer and mobile phone.
The advantage of defining HTML 5 in terms of the DOM is that the language itself can be defined independently of the syntax. There are primarily two syntaxes that can be used to represent HTML documents - the HTML serialisation (HTML 5) and the XML serialisation (XHTML 5).