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A user agent (UA) is a software element that acts on behalf of a user. However, the term user agent has also come to be associated with a header string in HTTP and related technologies that help a hosting server identify the type of operating system and/or device requesting content.
The use of user agents is a controversial topic because of UA requests for Web content and the creation of Web pages that handle various browsers in different ways. A movement to equalize access between browsers attempted to promote more broadly accessible design, but some sites are still designed with specific browsers or devices in mind. In general, the browser as UA is a key component of the Internet's developmental timeline. It continues to change as desktop and laptop computers give way to mobile devices for Internet viewing.
A broader UA definition includes Web crawlers, Web bots and other technologies. However, a Web browser is a simple example of a common UA setup. Here, the browser acts as the UA, where the user actively controls the browser. The browser issues certain UA strings that show the specific technology used to access a site, page or other content. For example, a UA string from a Firefox browser would include the word Mozilla, as well as version data and other details.