On the internet, an user agent is an application or service that renders or transforms webpage code, for example HTML.

The most common examples of user agents are web browsers, like Internet Explorer and Firefox, that render webpages into a visual and interactive representation in their browser window. However, user agents can also be search engine spiders such as Googlebot and Yahoo! Slurp (harvesting the code for keywords), screen readers such as JAWS and VoiceOver (rendering webpages into speach or tactile braille dots), email clients such as Thunderbird and Outlook (rendering HTML-formatted e-mails) and webmail clients such as Gmail and Hotmail (transforming HTML-formatted e-mail to HTML snippets that can be inserted into the HTML source of the webmail client).

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