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So I have this web-page in my local:
<html>
<head>
<title>Sample "Hello, World" Application</title>
</head>
<body bgcolor=white>
<table border="0">
<tr>
<td>
<img src="images/tomcat.gif">
</td>
<td>
<h1>Sample "Hello, World" Application</h1>
<p>This is the home page for a sample application used to illustrate the
source directory organization of a web application utilizing the principles
outlined in the Application Developer's Guide.
</td>
</tr>
</table>
<p>To prove that they work, you can execute either of the following links:
<ul>
<li>To a <a href="hello.jsp">JSP page</a>.
<li>To a <a href="hello">servlet</a>.
</ul>
</body>
</html>
And when I trace my HTTP Requests and Responses I will see:
GET /sample/ HTTP/1.1
Host: localhost:8080
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Encoding: gzip, deflate, lzma, sdch
Accept-Language: tr
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.65 Safari/537.36 OPR/26.0.1656.24
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 636
Content-Type: text/html
Date: Fri, 28 Nov 2014 19:48:47 GMT
ETag: W/"636-1185801988000"
Last-Modified: Mon, 30 Jul 2007 13:26:28 GMT
Server: Apache-Coyote/1.1
and second request is made for the image:
GET /sample/images/tomcat.gif HTTP/1.1
Host: localhost:8080
Accept: image/webp,*/*;q=0.8
Accept-Encoding: gzip, deflate, lzma, sdch
Accept-Language: tr
Referer: http://localhost:8080/sample/
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.65 Safari/537.36 OPR/26.0.1656.24
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1441
Content-Type: image/gif
Date: Fri, 28 Nov 2014 19:54:55 GMT
ETag: W/"1441-1185801988000"
Last-Modified: Mon, 30 Jul 2007 13:26:28 GMT
Server: Apache-Coyote/1.1
Why is the browser sending a Accept: image/webp,*/*;q=0.8
When I click on the image itself and open it in new tab the request will be sent as:
GET /sample/images/tomcat.gif HTTP/1.1
Host: localhost:8080
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Encoding: gzip, deflate, lzma, sdch
Accept-Language: tr
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.65 Safari/537.36 OPR/26.0.1656.24
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1441
Content-Type: image/gif
Date: Fri, 28 Nov 2014 19:55:53 GMT
ETag: W/"1441-1185801988000"
Last-Modified: Mon, 30 Jul 2007 13:26:28 GMT
Server: Apache-Coyote/1.1
Why is the second request has many more values in Accept header but in the first case it is different?
899
It's essentially a historical record of formats that browser manufacturers wanted to make it easy to identify support for.
As Grice points out, these all include */* and so accept anything; the specified formats are just preferences.
In at least one case:
Accept: image/webp,*/*;q=0.8
is simply a special case to promote the uptake of WebP. Modify Accept header to explicitly denote the image formats Chrome supports:
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
As Chrome clearly accepts PNG, JPEG and GIF as well, making WebP a special case is simply an attempt to encourage specific support for a preferred format. Likewise, the special case for application/xhtml+xml made a lot of sense when trying to encourage wider use of XHTML.
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