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Starred by 46 users

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Status: Verified
Owner:
Closed: Jun 2013
Cc:
Components:
EstimatedDays: ----
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OS: ----
Pri: 2
Type: Feature



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Modify Accept header to explicitly denote the image formats Chrome supports

Project Member Reported by sko...@chromium.org, Jan 10 2013

Issue description

As part of promoting the adoption of WebP, it would be useful if Chrome more explicitly indicated the image formats it supports in the Accept header.  Currently it just returns */*, but this makes it difficult for servers to know whether it's safe to return WebP images in lieu of JPEG or not (UA detection "works" but breaks down when proxy caches are involved).

Changing the Accept header in Chrome, combined with servers using a Vary : Accept on the response, would greatly streamline WebP adoption since servers could safely return WebP when the client can decode it and not be concerned that intermediate proxy caches will break things.

A full description of the problem and proposal is here:
https://docs.google.com/a/google.com/document/d/1l544msFFKdboPFEcBqSwZGlwidmR1756Ij_zGwkI-wQ/edit#

And a related discussion that yielded consensus on the change is here:
https://plus.sandbox.google.com/102108217417547514454/posts/SK28ChGeGmV


 

Comment 1 by sko...@chromium.org, Jan 10 2013

Cc: -ilya@google.com igrigo...@chromium.org

Comment 2 by sko...@chromium.org, Jan 11 2013

Labels: Area-WebKit

Comment 3 by engedy@chromium.org, Jan 21 2013

Labels: -Type-Bug Type-Feature Feature-Privacy
FYI for my team.

Comment 4 by engedy@chromium.org, Jan 23 2013

Labels: -Feature-Privacy
Labels: Mstone-27
Status: Available

Comment 6 Deleted

Project Member

Comment 7 by bugdroid1@chromium.org, Mar 10 2013

Labels: -Area-Internals -Feature-Browser -Area-WebKit -Mstone-27 Cr-Content Cr-UI-Browser-Core Cr-Internals M-27
This is definitely a blocker for us at Polar Mobile. We'd definitely love to start using WebP to serve images, when possible, from our APIs.

Comment 9 by urvang@google.com, Mar 13 2013

I created a one-line patch for Chromium which modifies the HTTP request header for all image requests to be "Accept: image/webp, */*;q=0.8" as suggested in the proposal, instead of the current "*/*". I checked the request headers for HTML & resources for some sample pages and it is working as expected. I also ran base_unittests and LayoutTests on the modified code and they all pass.

Can someone please review the same?

Important: There are a couple of WebKit changes that need to go in before this (support for some newer WebP features), so please DO NOT SUBMIT this now.
chromium_accept_header.patch
513 bytes View Download
Cc: jam...@chromium.org darin@chromium.org brettw@chromium.org tony@chromium.org

Comment 11 by darin@chromium.org, Mar 14 2013

Cc: abarth@chromium.org
It's not clear to me that we want to make this change.  There are two disadvantages:

1) The Accept header is very brittle, and we run the risk of breaking many sites.
2) Making this change bloats a large number of HTTP requests.  Once we add the bytes, it's much, much harder to remove them.

If you extrapolate where this process is heading, we'll end up with more and more bloat in HTTP requests and be unable to slim them down again.

IMHO, this patch is a workaround for the fact that WebP hasn't been adopted by other browser vendors.  A better place to spend our energy is in convincing other browser vendors to include WebP.

Comment 13 by skonig@google.com, Mar 14 2013

For #1, it's not clear to me that this is true; we aren't removing */*, just adding an explicit mention of WebP.  In any event, putting this change into Canary first and later Dev gives us an opportunity to validate this concern or not.  If it proves to break lots of sites we can easily revert it.

For #2, the # of bytes we're adding is quite small.  Compared to the number of bytes we save by making it easier to use WebP, the overall benefit is far in favor of this change.

I don't agree that this is a workaround for the fact that WebP hasn't been adopted by other browser vendors.  Even if everyone decided to adopt it tomorrow, the problem of legacy browser versions that don't support WebP would remain (e.g., older versions of IE).  UA detection is too unwieldy to work as a way to detect WebP support since it breaks proxy caches.  So we will always live in a world in which some clients don't support WebP; unless we want to throw up our hands and say we will never be able to adopt new image formats beyond those we have today, some solution to this problem is required.

And this effort is not in competition with energy in getting other browser vendors to support WebP; we're doing that too, aggressively so.  But this is a classic chicken-and-egg problem where browser support hinges on adoption and adoption hinges on browser support.  For reasons noted above we need to make adoption easier, recognizing we're in a world in which we'll never have 100% client support.  

This is absolutely the right change to make.  If you still disagree, let's meet to discuss further.

Comment 14 by tony@chromium.org, Mar 14 2013

Cc: tabatkins@chromium.org
FWIW, this is what the CSS image() function is supposed to solve:
http://dev.w3.org/csswg/css4-images/#image-fallbacks

WebKit doesn't currently implement image(), but it shouldn't be too hard to implement.

The benefits to using image() are:
a) Doesn't require web server changes.
b) It's similar to image-set(), which is already used for different resolution graphics.

It will also work fine with proxies, but doesn't work as well for the social sharing case.  At least for the social sharing case the URL can give a hint as to why it didn't work.

Comment 15 by ada...@gmail.com, Mar 14 2013

This improvement will bring Chrome into compliance with the HTTP spec http://tools.ietf.org/html/rfc2616#section-14.1  

It is unreasonable to expect people to maintain databases of which formats are accepted by which browser and browser version. Especially considering HTTP gives us a mechanism for determining which formats are accepted by the user agent.

Not only is this important for WebP, but it will make it easier to adopt any future standards that Google or others might propose. If we can use content negotiation and rely on it. Eventually the CDNs will start supporting content negotiation and rolling out a successor to WebP will be easier.

I don't think that the header should just say that WebP is accepted, but should also note its acceptance of png, jpeg, etc. If Chrome ever adds support for APNG or JPEG2000 then they should be added too.
Re #14: Is image() supported in all other browsers aside from WebKit?  If not then I'm not clear how this helps.  IIUC it would also require site authors to change all their HTML to use this instead of <img> tags, a non-trivial request.  

In contrast the Accept header change means much less churn and it's future-proof.  As more browsers support WebP they just indicate it in their Accept headers and no changes are required anywhere - it will just instantly start working.

Comment 17 by tony@chromium.org, Mar 14 2013

CSS's forward-compatible parsing rules make it possible to support browsers without image() support.  E.g.:
#logo {
  background-image: url('logo.png'); /* older browsers */
  background-image: image('logo.webp', 'logo.svg', 'logo.png');
}

It does require site authors to change their HTML, but that seems like a lower barrier to entry than requiring site authors to change how their webserver serves images.
So what will become the <img> HTML tag? That's the best tool for display images on browsers, where background-image CSS rule is better for design pictures but not for classic pictures like illustrations in an article. More, the <img> tag is SEO friendly with Google and other search engines.

I'm interested in the "accept header image/webp" because I have a PaaS as web hosting and the only way to implement WebP for me is to recognize the headers with PHP and serve images in jpg/png OR WebP with a function, and I don't want to work with all the user agents and string searching fonctions like : if Chrome > 3 or Opera > 9.10 or whatever, and that's a little insecure imo. And the javascript solution to display WebP images is heavy and tricky.

A webmaster point of vue.
I'll second skonig@'s comments:

Opera, FF, IE, send proper Accept headers, not */*. We are not breaking anything - we're unbreaking what webkit broke. In fact, Opera already sends image/webp in their Accept header, and by enabling it in Chrome, this eliminates the entire problem of UA-detection fallbacks required to deploy webp. 
 
Further, while image() is a welcome addition, it doesn't solve all problems: it's CSS only, and requires that you move everything into backgrounds -- good luck with that. That also means manual layout (must specify width/height for every image asset) and inability to interact with the image within the browser (drag and drop, right click to get URL, etc). At the very minimum, we need an identical mechanism within <img>. 

Last but not least, as Stephen already pointed out: this approach does not require that the entire web rewrites their markup. Instead, CDN's and caches can instantly opt-in users into appropriate formats the moment the support lands in the browser -- speaking of "making the web fast".. that's a massive win, especially for mobile UA's.
@skonig: Sounds like we need to discuss this topic further.

> For #1, it's not clear to me that this is true; we aren't removing */*, just adding an explicit mention of WebP.

The Accept header is brittle.  Even small tweaks have unpredictable consequences.

> In any event, putting this change into Canary first and later Dev gives us an opportunity to validate this concern or not.  If it proves to break lots of sites we can easily revert it.

Yes, we'd need to be careful about staging the change, but we should be able to de-risk the compatibility issues using this approach.

> For #2, the # of bytes we're adding is quite small.

That's true, but it's 100x easier to add a byte to every HTTP request than it is to remove a byte.  Every byte you add is small, but the cruft accumulates over time and bloats the platform.  The User-Agent header is a good example of this sort of cruft accumulating.

> Compared to the number of bytes we save by making it easier to use WebP, the overall benefit is far in favor of this change.

I don't think that's the metric you want to use to judge this feature.  The vast, vast majority of time you send these bytes on the wire, the server will simply ignore them.

IMHO, we should wait for the WebP line on chromestatus.com to have more green before we add bloat to every HTTP request.  In the end, what helped PNG adoption wasn't tricks with the Accept header, it was all the major browser vendors shipping PNG support.  The path to success for WebP is getting more browsers to support the format, not helping servers special-case Chrome.
@abarth: I set up some time on Monday for us to chat.  

>MHO, we should wait for the WebP line on chromestatus.com to have more green before we add bloat to every HTTP request.

I think that even if we get get every browser vendor to support it tomorrow, we're still going to need this change.  IE10, 9, 8 and 7 are still going to be around for a while, so we're always going to have to do content negotiation in some way, and every way other than the Accept header is unworkable.  But let's discuss further on Monday.
IE7-10 can be pretty easily identified with existing HTTP headers, can't they?
After discussion yesterday, we agreed to implement this change but apply it only to a whitelisted set of sites (*.google.com and selected 3rd party sites).  This will mitigate the potential risk of breaking lots of web sites due to a sudden change of Chrome's Accept header.  In the short-term we can add additional sites to the whitelist as they adopt WebP; in the medium term once additional browser support is available we will remove the whitelist entirely.

@abarth to point urvang@ to an example of where we've implemented whitelists like this in the past, then we can submit the patch.

Thanks!

Comment 24 by tomby...@gmail.com, Mar 19 2013

That is an interesting compromise for a whitelisted header change; thanks for implementing it!

Can this whitelist be user-adjusted please?  It would help webdevs to test how WebP would benfit their own site.  IMHO this could be key in snowballing acceptance, since many already use Chrome's Developer Tools.  If you are super-concerned about "breaking lots of sites", you could make the users' adjustments to be temporary (must be manually re-entered when Chrome is restarted).
cheers
@tomByrer: The approach we typically use is to have a command-line flag that enables the feature for all web sites.

Comment 26 by urvang@google.com, Mar 19 2013

@abarth:
Can you pls point me to a white-listing example?
@urvang: It's nothing sophisticated.  You just have an if-statement in the code that compares against a list of host names.
I 100% agree with  igrigo...@chromium.org...

> Last but not least, as Stephen already pointed out: this approach does not require that the entire web rewrites their markup. Instead, CDN's and caches can instantly opt-in users into appropriate formats the moment the support lands in the browser -- speaking of "making the web fast".. that's a massive win, especially for mobile UA's.

The Accept Header seems to be, hands-down, the best choice to start to see adoption/use-cases. I'll plead ignorance here, but I honestly don't understand why there's pushback on this implementation when other browsers already do this.

Comment 29 by urvang@google.com, Mar 24 2013

Based on the suggestion from abarth@, here is a patch which modifies the accept header only for a whitelist of domains.
Note that, this just has a sample whitelist to show this works. One could add more domains now/later.

Also, I'm looking at how to add a command-line option to enable this for all sites.
chromium_accept_header_whitelisted.diff
2.2 KB View Download
A command-line option will be VERY appreciated to enable devs to test the
site before it goes to prod.

Thanks for all your hard work, everyone!
Labels: -M-27 M-28
Moving to M28 as we likely won't get this done for M27.
FYI: The first patch for this is under review here:
https://bugs.webkit.org/show_bug.cgi?id=113260
After some further discussion, here's the plan of attack:

We'll add a flag (settable via the command line or about://flags) to include explicit mention of WebP in the Accept header.  The flag will be on by default for Canary but off by default for all other channels.  We'll use the intervening period to gather data about how this change performs in the wild, and what (if anything) a change like this breaks.  Sites can deploy this solution and test it for clients running Chrome Canary.  We expect this to land in the next week or so.

In parallel we're going to chat with Mozilla about a consistent Accept header across both FF and Chrome.  This will benefit proxy caches since they won't have to cache separately for both.

Once we've identified and resolved any unexpected side effects and resolved the proper Accept header we'll roll this out officially.
For record keeping:
- Current FF header: image/png,image/*;q=0.8,*/*;q=0.5
- Current IE header: image/png,image/svg+xml,image/*;q=0.8, */*;q=0.5 
- Proposed Chrome header: image/webp, */*;q=0.8
What now with Blink? Some changes are planned?
Blink doesn't change the equation for this.
Project Member

Comment 37 by bugdroid1@chromium.org, Apr 5 2013

Labels: -Cr-Content Cr-Blink

Comment 38 Deleted

Comment 39 by urvang@google.com, Apr 13 2013

FYI, I have added this command-line flag from Blink-side: https://chromiumcodereview.appspot.com/13814024/

The follow-up change (from Chromium side) would make it possible to enable-disable it from Chromium (work in progress).

Comment 40 by bruan...@gmail.com, Apr 16 2013

>> Compared to the number of bytes we save by making it easier to use WebP, the overall benefit is far in favor of this change.

> I don't think that's the metric you want to use to judge this feature.  The vast, vast majority of time you send these bytes on the wire, the server will simply ignore them.

I could not agree more. This bugs is about adding waste to probably 99% image HTTP request on the web.
If we ever reach a point where all browser vendors support WebP and older browsers not supporting it die off (as it's the case with JPEG and PNG), then web devs will use webP anyway regardless of any header. Removing later the webp format from the Accept list will result in either broken websites or slower websites which no one wants.
Any change to the Accept header should be temporary and web browser game theory tells us it cannot be temporary.

It breaks my heart to say it, but for the transition period (during which some browsers support WebP and others don't) you should be recommanding authors to do server-side user-agent sniff. It's as weak as the Accept header, but at least you're not adding more junk to the web.


I also highly recommand to read this article about why content-negociation doesn't work: http://wiki.whatwg.org/wiki/Why_not_conneg
To prevent reiterating mistakes of history, please also read https://bugzilla.mozilla.org/show_bug.cgi?id=824623 and the different older bugs it links to.


> In parallel we're going to chat with Mozilla about a consistent Accept header across both FF and Chrome.  This will benefit proxy caches since they won't have to cache separately for both.

Did this happen? What has the outcome been?

Comment 41 by jbhol...@gmail.com, Apr 16 2013

If even 1% of image requests use WebP, this seems like a savings to me. I think this should be carefully considered with more concrete information. What I can offer is one real-world example: https://secure.kalmbach.com/order/?c=IMRV&ch=ADDMRV

A cold load of that page with non-WebP images is ~273KB. A cold load of that page with WebP images is ~132KB. It would take over 8000 wasted image requests to equal the savings from a single cold load of this site. The source images are highly optimized, and with 9 directly-hosted images, it's obviously not a graphically intensive site. Other sites can only benefit more.

I would seriously request an attempt to study the state of WebP now, and attempts to get buy-in from CDNs - probably the fastest way to boost WebP support - before saying yes or no to a header change.

Some other considerations:
Beyond the comparison of raw data sizes, aren't bytes relatively cheap compared to packets, especially on mobile?

How much will 18 extra bytes in the header compress to when SPDY/HTTP2 is more common?

Proxy caching will be a lot smoother with a header than with user-agent sniffing. If we're forced to do user-agent sniffing and proxy caching is effectively disabled, it may ultimately negate the savings of WebP.

Comment 42 by bruan...@gmail.com, Apr 17 2013

> I would seriously request an attempt to study the state of WebP now, and attempts to get buy-in from CDNs - probably the fastest way to boost WebP support - before saying yes or no to a header change.

Agreed. I threw 99% randomly, it could be much more. It would be worthwhile doing a study before changing the headers. This study could also try to assess which amount of website do deliver different content based on the Accept header.


Even if you add "webp" to Accept, web devs won't be able to do the economy to test which of webp is actually supported. You can play around with the "q" parameter, but it doesn't change anything in the testing part and maintaining the "Accept heuristic".

Also, if a point is reached when all browsers do support webp, then the header value will just be a pure waste as it currently is with png in Firefox. As I linked to in my previous message, keeping "image/png" isn't a choice by Mozilla. It's imposed on them because removing it would break websites. If you do change your Accept header, you're facing the *exact same* situation.
The header change really is a temporary thing that will tie you


> Beyond the comparison of raw data sizes, aren't bytes relatively cheap compared to packets, especially on mobile?

I did not understand this point. But at scale every byte counts. Especially bytes that can't ever be removed.


> How much will 18 extra bytes in the header compress to when SPDY/HTTP2 is more common?

If SPDY/HTTP2 was deployed today, I would be less worried about it, that's a very good point. But we're not there yet and I don't think anyone really knows when it will all be deployed at scale. As long as a majority of deployed servers are HTTP1.1, this cannot be a valid argument.


> Proxy caching will be a lot smoother with a header than with user-agent sniffing. If we're forced to do user-agent sniffing and proxy caching is effectively disabled, it may ultimately negate the savings of WebP.

It's ugly, but "Vary: User-Agent" could work (it only fails if an old browser pretends to be a new webp-supporting browser. Rare situation admittedly). Once again, that's all for the temporary solution where webp is supported in some browsers and not others. 
I'm not an image expert, but I read enough enthusiasm about webp based on technical merit that I'm pretty sure it'll be supported everywhere eventually.

Comment 43 by urvang@google.com, Apr 18 2013

Coming back to the point:
The change from the Chromium side is in as well:
https://chromiumcodereview.appspot.com/14273007/

Now, interested folks should be able to test it out -- by enabling it from the command-line (--enable-webp-in-accept-header) or through 'chrome://flags'.
Project Member

Comment 44 by bugdroid1@chromium.org, Apr 18 2013

------------------------------------------------------------------------
r194853 | urvang@google.com | 2013-04-18T08:35:15.089552Z

Changed paths:
   M http://src.chromium.org/viewvc/chrome/trunk/src/content/public/common/content_switches.cc?r1=194853&r2=194852&pathrev=194853
   M http://src.chromium.org/viewvc/chrome/trunk/src/content/public/common/content_switches.h?r1=194853&r2=194852&pathrev=194853
   M http://src.chromium.org/viewvc/chrome/trunk/src/chrome/browser/about_flags.cc?r1=194853&r2=194852&pathrev=194853
   M http://src.chromium.org/viewvc/chrome/trunk/src/content/renderer/render_thread_impl.cc?r1=194853&r2=194852&pathrev=194853
   M http://src.chromium.org/viewvc/chrome/trunk/src/content/browser/renderer_host/render_process_host_impl.cc?r1=194853&r2=194852&pathrev=194853
   M http://src.chromium.org/viewvc/chrome/trunk/src/chrome/app/generated_resources.grd?r1=194853&r2=194852&pathrev=194853

Add a switch to Chromium to enable/disable 'image/webp' accept header.

This switch is off by default. It will be turned on later once we verify that this change doesn't break any sites.

It can be can be enabled through command-line (--enable-webp-in-accept-header) or through 'chrome://flags'.

Note: This is a follow up of the pre-requisite change in Blink: https://codereview.chromium.org/13814024/

BUG= 169182 

Review URL: https://chromiumcodereview.appspot.com/14273007
------------------------------------------------------------------------
For anyone want to test this functionnality and have a shared hosting like me, you can do anyway with Apache and some htaccess rules (the same are adaptable for Nginx I guess), because the Accept header trigger only on images.

Here the rules: http://pastebin.com/Qgx2X974

Simply put your png/jpg/gif files in a images/classic folder, and webp equivalents (with same names) in a images/webp folder. The rules are simple and you can change them with ease. The rules are PageSpeed-proof (I get 100/100 with them). You don't need to have all webp equivalent images: if one is missing, the conditions is not met so the rule don't apply, and the classic image is displayed.

Everything is fine with this technique, and it works with Opera which support webp with an Accept header too.
Project Member

Comment 47 by bugdroid1@chromium.org, Apr 30 2013

------------------------------------------------------------------------
r197474 | skonig@chromium.org | 2013-04-30T21:22:52.053103Z

Changed paths:
   M http://src.chromium.org/viewvc/chrome/trunk/src/content/browser/loader/resource_dispatcher_host_impl.cc?r1=197474&r2=197473&pathrev=197474
   M http://src.chromium.org/viewvc/chrome/trunk/src/tools/metrics/histograms/histograms.xml?r1=197474&r2=197473&pathrev=197474

Add histogram for image errors.
This CL adds a histogram for errors loading images.  It will be 
used to measure how changes impact image request responses.
 
BUG= 169182 

Review URL: https://chromiumcodereview.appspot.com/14093027
------------------------------------------------------------------------
Project Member

Comment 48 by bugdroid1@chromium.org, May 8 2013

Labels: -M-28 MovedFrom-28 M-29
Moving all non essential bugs to the next Milestone.
Labels: -MovedFrom-28 -M-29 M-28
Owner: sko...@chromium.org
This is in progress for M28.
Status: Fixed
I see Chrome sending image/webp in the Accept header for image requests, but at least for my purposes that's not enough. Our static image assets are stored on S3, which won't do things like content negotiation. While I could force image requests to hit my app server first, and either be proxied or redirected to the appropriate S3 endpoint, I'd rather not add that extra overhead. Ideally Chrome would send image/webp in the Accept header for all requests so that our backend can make the <img src> determination when generating the page's HTML.
Status: Verified
@51: Good point.  We opened a new bug to fix that, see  Bug 267212 .
Thanks! I'm seeing the image/webp header for http requests now too. That's awesome! One last place I'd add it to are CSS requests. That way I can server push webp images along with a CSS request.

Comment 54 by rog...@inventid.nl, Aug 31 2015

Chrome on iOS has problems fetching json from some APIs based on this header... http://stackoverflow.com/questions/32287347/why-does-chrome-for-ios-insert-image-webp-content-type/32318334#32318334
rogier: the linked issue reports two different headers ("Accept: image/webp, */*;q=0.8", and "Accept: */*,image/webp"). Can you provide a netlog trace of the XHR request?

Also, if you want to fetch JSON specifically, you can set the Accept header as part of the XHR (which will override the UA default).
Our own company API faces this issue, you can try it by going to https://shop.inventid.nl/1. It will do a XMLHttpRequest to `/shops/1`, which will fail with a 415
Testing on latest Chrome for IOS I see it sending "accept: */*,image/webp", which is a valid value. The fact that the server is rejecting it is a server bug -- the client is advertising what it supports, the server shouldn't reject that. 

Also, if the intent is to fetch JSON via XHR, then you should indicate that via Accept -- e.g. xhr.setRequestHeader('Accept', 'application/json').

Comment 58 by Deleted ...@, Sep 11 2015

It seems Chrome for iOS alters the Accept header in ajax requests and append images/webp even when the Accept header is set, that causes some problems.

Comment 59 by ch...@orr.me.uk, Nov 13 2015

Chrome/48.0.2560.0 (current canary) is now sending "Accept: */*" for image requests.

Chrome/47.0.2526.58 (current beta) and stable builds are sending the expected "Accept: image/webp,image/*,*/*;q=0.8" header for image requests.

This breaks the WebP support I have deployed via content negotiation.


Note that Chrome 48 is still claiming support for WebP when requesting web pages:
"Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"

So it's unclear why it's no longer sent for image requests?


(I note that CSS requests now also use "*/*" rather than "text/css,*/*;q=0.1", so I guess this is a more general change (or side-effect) in Accept behaviour?)

Comment 60 by jzern@chromium.org, Nov 14 2015

I opened another bug to track the comment in #59.

https://code.google.com/p/chromium/issues/detail?id=555914

Comment 61 by mich...@layer.com, Dec 30 2015

I am also observing what #58 reported; we explicitly set our accept headers but chrome for IOS adds image/webp to our accept header.  The browser may support image/webp, but my xhr request does not, and it should not be a mandatory part of the accept header.

Comment 62 by urvang@google.com, Dec 30 2015

@Michael: Can you please create a new bug with all the details at crbug.com/new?
Thanks.

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