Thanks for your input and, as a general response to your post, please see Additions to Approved HTML.
I was aware that ids needed to be unique but thought that these would be resolved by Node IDs (assuming authors used unique IDs in their individual nodes); however, upon further consideration, that's probably little more than wishful thinking.
ambrus (in Re^4: Approved PM markup?) points out issues with using CSS in posts:
I've dropped the suggestion of using either the style element or style attributes.