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Re: Encoding interaction of HTTP response header and META tag

From: Wayne pollock <wpollock@hccfl.edu>
Date: Sat, 5 Mar 2011 02:50:06 -0500
Message-Id: <383E8906-DC82-4F15-ACCA-2FD964058885@hccfl.edu>
To: Harley Rosnow <Harley.Rosnow@microsoft.com>
Cc: "Jukka K. Korpela" <jukka.k.korpela@kolumbus.fi>, "public-html-comments@w3.org" <public-html-comments@w3.org>, Adrian Bateman <adrianba@microsoft.com>
Thanks for the informative replies.  I see now it cannot be changed.  But, I would like to see a note added to this part of the standard, explaining the security rational.

As was noted, Apache only allows one charset per extension easily.  Adding .htaccess files must be done carefully to avoid security issues.  And it really does have a huge impact on Apache performance; every access must check every directory on the file's path for .htaccess files---that info isn't cached as far as I know.

But security is vital, so I agree it is not wise to change the order.  I guess the best procedure is to convert all files on a server to a single encoding.  (While I like UTF-8 too, I can see where that is biased against non-Western languages and no single byte encoding will work well for everyone.) 

Wayne Pollock

On Mar 4, 2011, at 7:41 PM, Harley Rosnow <Harley.Rosnow@microsoft.com> wrote:

> When I first encountered this order of precedence in HTML, I had the exact same response as you, Jukka.  In general, when designing software systems, I like to give higher precedence to information that's closer to the content which it describes.  Your points about the efficiency and difficulty of managing HTTP response headers vs. the META tag (and the BOM for UNICODE files) are well taken.  But,  I still think the spec should not change.
>> Wayne Pollock wrote:
>>> If document authors
>>> goes to the trouble of stating the charset in the HEAD of their 
>>> document, that that should override any default set by the web sever.
>> I have much sympathy for the idea, for reasons you gave, especially 
>> the reason that web server admins often disallow the effects of 
>> .htaccess files, effectively enforcing their settings on every authors.
>> However, I'm afraid it's too late; the change would break a long 
>> tradition and would break existing pages.
> Yes, it would cause compatibility issues to change this.  However, that's not the reason to keep it.
> The reason for this precedence order is security against XSS attacks.  While hackers can sometimes insert content into web pages or even the very beginning of the stream in some vulnerabilities, it's much more difficult for hackers to manipulate HTTP headers.  In many cases I've encountered vulnerabilities completely mitigated by the presence of an HTTP header specifying the encoding of the web page (or XHR response).  The order of precedence of the HTTP header is essential for security and that's the reason we *really* cannot change it.
> The other reason I've encountered for this order of precedence is that it allows for the transcoding of the content as it moves through proxy servers and other network entities between the client and the server.  I've not really encountered this in my work, but that's doesn't mean it's not happening on the Web.
>>> It is a huge
>>> burden to webmasters everywhere to have to manually set the charset 
>>> for every update to their website.
>> I can't see what you mean by that. The settings need to be checked 
>> when you start creating a site, not after every update.
> Yes, I agree.  It's often trivial to associate an HTTP header with a particular type of file and difficult to set a different header for each file.  Often, files are created by different authors and the encoding of the file can be changed by selecting a different option in a tool by the author.  At the same time, getting the HTTP header into sync with the author can require the web administrator to make the change.  This precedence scheme can cause an increasing burden as the scale of the web site and organization grows, if multiple encodings are supported.
> My one recommendation here is to require the use UTF-8 everywhere.  This approach scales up well with minimal burden.
>> Pardon? Apache settings operate per filename extension, and mostly it 
>> suffices to set the encoding for just one extension, ".html".
>>> Such overriding is possible but
>>> to allow web authors the ability to do so, per directory settings 
>>> must be  enabled (the ".htaccess" files).  doing so severely impacts 
>>> server performance
>> I don't think it has any significant impact on performance.
>>> and many sites simply can't do so, so web pages WILL be send with 
>>> the wrong charset.
>> Well, I would put it this way: If the server admin disallows the 
>> effects of your .htaccess file, then it's just something you need to 
>> live with it. If the force your HTML documents to be served with 
>> headers saying that the encoding is iso-8859-1, or utf-8, or whatever, 
>> then just make it so
> I've more experience with IIS than Apache, but web authors need to live within the requirements set by their web admins.  
>>> This should be a simple fix.  The issue was raised on the WHATWG 
>>> list and elsewhere, and noboby could think of an objection to this 
>>> proposal.
>> I think a more specific citation of previous discussions would be needed.
>>> (The
>>> only web pages that could "break" with this change were already
>>> broken.)
>> 99% of web pages are broken, in the sense of not complying with HTML, 
>> CSS, WCAG 1.0, or other relevant recommendations. When we worry about 
>> what happens to existing pages, we need to worry about more or less 
>> broken pages, mostly.
>> Consider a page on a server that forces Content-Type: text/html;
>> charset=utf-8 on all HTML files. Such servers are increasingly common.
>> Authors have had to accommodate to that, for example saving documents 
>> in
>> utf-8 encoding if needed. The pages may well have <meta> tags 
>> announcing
>> iso-8859-1 or something else, maybe because some web page editing 
>> software emitted it, or it belonged to a sample file used as a 
>> starting point, or the author copied it from somewhere, with little or 
>> no understanding of its effect.
>> Your proposal, if accepted and implemented, would imply that all such 
>> pages stopped working, if they (literally) contain any character 
>> outside the ASCII range. This might mean a mess that everyone can see, 
>> or just one character might be wrong, or anything between these extremes.
>>> On a related note, the new structural tags that denote articles and 
>>> such should allow an optional CHARSET attribute.  A web page with 
>>> ARTICLEs etc. may be (and may likely be) composed of content from 
>>> many sources, e.g., a "mash-up".  While CMS and blogging software 
>>> could force a single charset so there is only one charset per web 
>>> page, that seems an unnecessary restriction (and I don't know that 
>>> most blogging software works that way).
>> No, it's an inherent restriction. The idea of allowing different 
>> character encodings within a single document has often been suggested, 
>> but it's based on a misunderstanding. Changing the encoding at a 
>> higher protocol level conflicts with the basic modern model of using 
>> character data. Recognizing encoding from meta tags is admittedly in 
>> conflict with it, too, but it was a more or less unavoidable 
>> exception, which has been separately defined (and is still known to 
>> cause problems, especially when people don't understand how it works 
>> and place it too late in the document). - "Mash-up" simply needs to recode when needed.
> I agree with Wayne that multiple encodings within the same file must be rejected.  Thanks,
> Harley Rosnow
> Internet Explorer
> Microsoft Corporation
Received on Saturday, 5 March 2011 07:57:25 UTC

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