HTML has served the Web community well, but it lacks some basic semantics necessary for modern Web applications. HTML 5 attempts to rectify this, while at the same time updating the HTML specifications to address issues raised in the past few years.
HTML 5 is still in the draft stage and is undergoing heavy development. Major changes in the specification can be expected.
The "class" attribute in HTML 5 has some predefined values, but it can also be extended to support other class names. Those values may be registered on the ClassExtensions wiki page, though registration is not necessary to use non-standard values.