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The expansions of Ω and Å are prohibited in NFC. NFC normalises them to U+03A9 and U+00C5. Perhaps HTML5 should make these entities align with NFC. http://krijnhoetmer.nl/irc-logs/whatwg/20080725#l-369 ohm; U+03A9 angst; U+00C5 (i.e. same as Ω and Å)
HTML5 just uses whatever the MathML WG decides. I recommend taking it up with David Carlisle <davidc@nag.co.uk>, who maintains the source file.
Adding myself to the cc list for this bug. (On the face of it the suggested change looks correct, but I need to think a bit about implications on systems that have made use of the existing (longstanding) mappings. So I'll delay deciding whether or not not to make a change. David
I made this change in the editors draft for the xml entities spec http://www.w3.org/2003/entities/2007doc/overview.html#d2e177 I believe the same unicode.xml source change will affect the html5 definitions.