BBcodes B and I return <strong> and <em> tags instead of CSS under inherited styles

Hi,

Please, refer to the topic : http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=50486

Summary of this issue:
The BBCodes b and i respectively return <strong> and <em> tags instead of returning a <span> tag with CSS, when the style inherit from prosilver,

How to reproduce this bug?
See attached screenshots