First off, Heather linked to my ramblings about how I made my own little Mirror Project widget.
Then I got an email from Aaron:
I saw your mention that the xml-rpc widget wouldn’t pass validation which struck me as a bad thing, so I’ve updated the code and everything should be good now."
Sure enough, the updated widget now validates as XHTML strict.
I’ve decided to keep my simplified little widget for now. I can customise the appearance of it more.
But I did install the XML-RPC libraries and here’s the working, validating Mirror Project widget: