Euro sign in HTML

According to Google Doctype € is supported in all browsers. But maybe not all fonts.

edit I also agree with Konrad Rudolph, use the character directly if you can.

Which one would you use in HTML?

None of them. Use an appropriate encoding (i.e.: UTF-8 or another Unicode transform) and use the charcter directly. Do not use HTML entities if at all avoidable, since they’ve got no advantage over use of a proper encoding.

Also, this is wrong:

It's a pity that there is no standard way to display the sign.

There is, and it’s the way I’ve described. Literally every browser, down to and including MSIE 5 will display Unicode characters correctly if the chosen fond supports the glyph.

The only valid reason to not use Unicode characters and instead fall back to entities might be projects that use legacy software which doesn’t support Unicode well. But that should never happen, right?