Adding a custom tag to sgml files

How would I go about adding a custom tag to the docs, and then control what
the html processor outputs for it?

E.g. Suppose I wrap some text in one of the sgml files with a <mytag
class="foo" anotherattr="bar">, </mytag> and then want to have that
translate to a <span class="foo" anotherattr="bar">, </span> in the html
output.

I realize this is more of a DocBook question and not specific to the pg
docs, but hoping something familiar with the underlying tools (which I've
spent the better part of today trying to learn, with limited success) has
the answer. Thanks.