浏览器如何确定Accept标头？

Koray Tugay 发表于 Dev

科雷旅

所以我在本地有这个网页：

<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>

当我跟踪HTTP请求和响应时，我将看到：

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

并再次请求该图像：

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

浏览器为何发送 Accept: image/webp,*/*;q=0.8

当我单击图像本身并在新选项卡中将其打开时，请求将以以下方式发送：

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

为什么第二个请求在Accept标头中有更多的值，但在第一种情况下却有所不同？

乔

它本质上是浏览器制造商希望使其易于识别支持的格式的历史记录。

正如格赖斯指出的那样，这些都包括在内*/*，因此可以接受任何东西。指定的格式仅是首选项。

在至少一种情况下：