[whatwg] Encoding sniffing algorithm - update proposal
Leif Halvard Silli
xn--mlform-iua at xn--mlform-iua.no
Thu Jul 26 15:27:48 PDT 2012
I have just written a document on how implementations prioritize
encoding info for HTML documents. (As that document shows, I have
not tested Safari 6.) Based on my findings there, I would like to
suggest that the spec's encoding sniffing algorithm should be updated
to look as follows:
Revised encoding sniffing algorithm proposal:
NEW! 0. document is XML format - opt out of the algorithm.
[This step is already implicit in the spec, but it would
make sense to explicitly include it to make sure that
one could e.g. write test cases to see that it is step
is implemented. Currently Safari, Chrome and Opera do
not 100% implement this step.]
NEW! #. Alternative: The BOM signature could go here instead of
in step 5. There is a bug to move the BOM hereto and make
it override anything else. What speaks against this are:
a) that Firefox, IE10 and Opera do not currently have
b) this revision of the sniffing algorithm, especially
the revision in step 6 (required UTF-8 detection),
might make the BOM-trumps-everything-else override
What speaks for this override:
a) Safari, Chrome and legacy IE implement it.
b) some legacy content may depend on it
1. user override.
(PS: The spec should clarify whether user override is
NEW! 2. iframe inherits user override from parent browsing context
[Currently not mentioned in the spec, despite that "all"
UAs do have this step for HTML docs.]
3. explicit charset attribute in Content-Type header.
4. BOM signature [or as the second step, see above]
5. native markup label <meta charset=UTF-8>
NEW! 6. UTF-8 detection.
I think we should separate UTF-8 detection from other
detection in order to make this step obligatory.
The newness here is only the limitation to UTF-8
detection plus that it should be obligatory.
(Thus: If it is not detected as UTF-8, then
the parser proceeds to next step in the algorithm.)
This step would make browsers lean more strongly
NEW! 7. parent browsing context default.
The current spec does not mention this step at all,
despite that both Opera, IE, Safari, Chrome, Firefox
do implement it.
Regarding 6. and 7., then the order is important. Chrome
does for instance perform UTF-8 detection, but it does it
only /after/ the parent browsing context. Whereas everyone
else (Opera 12 by default, Firefox for some locales - don't
know if there are others) let it happen before the 'parent
browsing context default'.
NEW! 8. info on “the likely encoding”
The main newness is that this step is placed _after_
the (revised) UTF-8 detection and after the (new) parent
browsing context default.
The name 'the likely encoding' is from the current spec
text. I am a bit uncertain about what it means in the
current spec, though. So I move here what I think make
sense. The steps under this point should perhaps be
a. detection of other charsets than UTF-8
(e.g the optional Cyrillic detection in
Firefox or legacy Asian encoding detection.
The actual detection might happen in step 6,
but it should only be made to count here.)
b. markup label of the sister language
<?xml version="1.0" encoding="UTF-8"?>
(Opera/Webkit/Chrome currently have this directly
after the native encoding label step - step 5.
c. Other things? What does "likely encoding" current
refer to, exactly?
9. locale default
 To the question of whether the BOM should trump everything else,
then I think it it would be more important to get the other parts of
this algorithm right. If we do get the rest of it right, then the 'BOM
should trump' argument, becomes less important.
Leif Halvard Silli
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