Unicode 5.2 -> 6.0

Gervase Markham gerv at mozilla.org
Fri Oct 15 16:42:55 CEST 2010

On 15/10/10 15:31, John C Klensin wrote:
> As a matter of principle and theory, I wonder about this and
> would like it if you explained it a bit further.  Keep in mind
> that IDNA2003 essentially treats unregistered code points as
> valid on lookup.   And, as far as I know, most browsers today
> permit me to install and use my one fonts, at least for the
> purpose of rendering content.   So, if you encountered one of
> these code points, encoded in an A-label, in a pre-IDNA2008
> version of the browser, in a deep subdomain of a TLD you
> considered careful enough, would you not decode the A-label to a
> U-label and then pass it off to whatever does rendering... and
> which might include a pass through a font set (or a
> locally-modified system font) that you didn't actually know
> anything about?

Actually, you are quite right. I should have thought more carefully. If 
someone were to register foo.org, and then create U+19CA.foo.org, and if 
the user had a font installed which had a glyph for that character, it's 
possible that Firefox would display it.

Whether it would or not depends on our algorithms for choosing which 
font to use in the URL bar and other parts of the UI, algorithms with 
which I am not familiar. I would not be surprised if we do _not_ allow 
arbitrary font fallback, because the URL bar has to remain readable for 
security reasons. (Say, for example, the only available glyph was in a 
cursive script font - would we choose it?) It could be that we limit the 
fonts which Firefox may select. But I cannot state with certainty that 
this is so.

I still stand by my conclusion that the likelihood of going with Unicode 
6.0 in its entirety causing a problem is very slight.

> The disadvantage is that
> every use of that table implies a character that might need to
> be treated slightly differently in IDNA then it would be in
> running text or other i18n applications.

Quite so. You put it better than I did.


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