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.

If id were allowed, that would allow id collisions, ... could be used for attribution confusion, navigation hazards and other tricky annoyances

Really? I mean, really, really!?

I know that lore has it that duplicate ids in html is verboten -- although HTML5 relaxed the specification to:

The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character

with duplicates in a given subtree being ignored. -- but is the possibility of duplication -- accidental or deliberate -- so heinous that we must prevent it at all costs?

Isn't the worst that could happen is that the duplicates might both get styled the same way?

I could (vaguely) understand the restriction if this place was a JQuery-driven game-show site, like so many are these days; but as-is, and seems likely as-always-will-be, isn't the banning of ids just another piece of over-zealous control-freakery?

What are you hoping to achieve with an ID?