The <emphasis> tag is rendered by the default HTML stylesheet as italic (<I>). If you want to change the appearance you have to do this in your customization layer. Jirka Kosek proposed the following solution recently on DocBook-Apps ML:
(element emphasis
(if (equal? (normalize "bold") (attribute-string (normalize "role")))
($bold-seq$)
($italic-seq$)))This solution looks at the role attribute and uses the <B> HTML tag to render <emphasis> if this attribute is set to bold.