Almost every tbook element can have a style and/or a
class attribute. This is also true for HTML, and in both formats
they have the same meaning. Thus their values are passed without any
changes to the HTML file, so you can use that for layout changes in the
HTML output.
The class attribute takes the name of a layout class. This is
actively used in a couple of tbook elements, e.g. in
<theorem>, but also in the top-level elements, see LaTeX sty file. However you can make much more use of it in the HTML output.
The style attribute takes CSS commands, see CSS.
Unfortunately it is impossible to preserve this information in DocBook,
and only very few CSS commands can be sensibly translated in LaTeX.
Just one of them is indeed realised so far, namely text colour.