Editing User-Agent header (section)

=== Format for human-operated web browsers ===

The user agent string format is currently specified by section 10.1.5 of [https://www.rfc-editor.org/rfc/rfc9110.html#name-user-agent HTTP Semantics]. The format of the user agent string in HTTP is a list of product tokens (keywords) with optional comments. For example, if a user's product were called WikiBrowser, their user agent string might be ''WikiBrowser/1.0 Gecko/1.0''. The "most important" product component is listed first.

The parts of this string are as follows:

* product name and version (''WikiBrowser/1.0'')
* layout engine and version (''Gecko/1.0'')

During the first [[browser wars|browser war]], many web servers were configured to send web pages that required advanced features, including [[Framing (World Wide Web)|frames]], to clients that were identified as some version of [[Mozilla]] only.<ref>[http://webaim.org/blog/user-agent-string-history/ History of the browser user-agent string]. WebAIM.</ref> Other browsers were considered to be older products such as [[Mosaic (web browser)|Mosaic]], [[Cello (web browser)|Cello]], or [[Samba (web browser)|Samba]], and would be sent a bare bones HTML document.

For this reason, most Web browsers use a user agent string value as follows:

{{block indent|Mozilla/[''version''] ([''system and browser information'']) [''platform''] ([''platform details'']) [''extensions'']}}

For example, Safari on the iPad has used the following:

 Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405

The components of this string are as follows:

* ''Mozilla/5.0'': Previously used to indicate compatibility with the Mozilla rendering engine.
* ''(iPad; U; CPU OS 3_2_1 like Mac OS X; en-us)'': Details of the system in which the browser is running.
* ''AppleWebKit/531.21.10'': The platform the browser uses.
* ''(KHTML, like Gecko)'': Browser platform details.
* ''Mobile/7B405'': This is used by the browser to indicate specific enhancements that are available directly in the browser or through third parties. An example of this is Microsoft Live Meeting which registers an extension so that the Live Meeting service knows if the software is already installed, which means it can provide a streamlined experience to joining meetings.

Before migrating to the [[Chromium (web browser)|Chromium]] code base, [[Opera (web browser)|Opera]] was the most widely used web browser that did not have the user agent string with "Mozilla" (instead beginning it with "Opera"). Since July 15, 2013,<ref>{{cite web|url=http://my.opera.com/ODIN/blog/2013/07/15/opera-user-agent-strings-opera-15-and-beyond |title=Opera User Agent Strings: Opera 15 and Beyond |publisher=dev.opera.com |date=15 July 2013 |access-date=2014-05-05}}</ref> Opera's user agent string begins with "Mozilla/5.0" and, to avoid encountering legacy server rules, no longer includes the word "Opera" (instead using the string "OPR" to denote the Opera version).