With due credit to Brownedov, the following notes are archived here.
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Good luck - ed - Sat Aug 16 12:48:20 BST 2008

This post is the result of my experimenting here and elsewhere with HTML using the new blog parser. It isn't exhaustive and is obviously subject to change if the BBC techies fix known issues.

HTML coding that works on BBC Blogs Special Characters (HTML Entities)

HTML character formatting:

HTML Links <a href="http://news.bbc.co.uk/1/hi/uk_politics/">BBC Politics</a> will give this: BBC Politics, but every & in the URL must be replaced with &amp; to get past the parser.

<ul> for a group of bulleted paras:

<blockquote> for indented quotations:

Oh what a tangled web we weave when once we practice to deceive.

The <br> Tag is normally used alone in HTML as the only common exception to the rules on closing Tags. In BBC blog comments, the CrLf key [Enter] or [Return] generates an automatic <br> by the parser. However, <br> does work if you want it to, but has to be matched with a normally meaningless </br> Tag.
This line was entered
without line breaks but
appears as multiple lines
by using <br> and </br> Tags with no whitespace between them.

The <p> Tag is normally used in HTML to separate paragraphs of text and is accepted by the parser. As always, it has to be matched with a </p> Tag.

This line was entered

<p>without line breaks but

<p>appears as multiple paragraphs

<p>by using <p> and </p> Tags.

Remember, every <html tag> needs to be closed with a corresponding </html tag> working from in to out. So to get bold and italic, you need <b><i>bold italics</i></b>

HTML coding that doesn't work on BBC Blogs
Special Characters (HTML Entities) Only the entities shown above seem to work. Even &nbsp; for extra white space produces an error. HTML special characters above &#127; don't produce an error but neither do they work, just giving a square:"�" or a Question Mark:"?", so you cannot use pound signs or accented characters.
HTML formatting the parser ignores:

If your post is accepted, you should see it as the most recent post on the thread - usually awaiting moderation - unless there are more than 500 posts on the thread when you get dumped back to the first page.
If your post contains HTML Tags the parser can't deal with, it will just ignore them but the formatting of the resulting post will be, at best, unattractive.
If your post contains an HTML Entity the parser can't recognise, you'll see "dnaerrortype=XmlParseError" in the URL the browser goes to after you press "Post Comment". You can usually recover the text to re-edit it by pressing the "Back" button on your browser.
For safety, especially if your post is a long one, copy it to the clipboard before attempting to post it.

Valid HTML 4.0 Transitional