about: is an internal URI scheme (also known as a "URL scheme" or, erroneously, "protocol") in various web browsers to display certain built-in functions and Easter eggs. It is not an officially registered scheme, and has no standard syntax.

In early versions of Netscape, any address beginning about: which wasn't recognised as a built-in command would simply result in the text after the colon being displayed. Similarly, in early versions of Internet Explorer, about: followed by a string of HTML (e.g. about:<em>hello world</em>) would render that string as though it was the source of the page - thus providing a similar (though more limited) facility to the data: URI scheme defined by RFC 2397. Still other versions of Netscape would return various phrases in response to an unknown about: address, including "Whatchew talkin' 'bout, Willis?" (a catch phrase from the TV show Diff'rent Strokes) or "Homey don't play dat!" (from a recurring skit on the TV show ''In Living Color'').

The most commonly implemented about: URIs are about:blank, which displays a blank HTML document, and simply about:, which may display information about the browser. Opera uses URIs beginning with opera: for similar purposes, and many about: URIs will be translated into an appropriate opera: URI if entered; an exception is about:blank, which is not translated.

Common about: addresses

\|+Common about: addresses ! Addresses ! Mozilla/SeaMonkey ! Mozilla Firefox ! Opera ! Internet Explorer ! Konqueror \|- \| about: \| colspan="2" \| Shows version information and copyright information \| Translated to "opera:about" \| \| Translated to "about:konqueror" \|- \| about:about \| Lists all the about: addresses \| \| Translated to "opera:about" \| \| Translated to "about:konqueror" \|- \| about:blank\| colspan="5" \| Shows a blank HTML document \|- \| about:plugins \| colspan="2" \| Shows the filename, MIME type, description, suffix, and status of all installed plug-ins \| Translated to "opera:plugins" \| \| Shows the filename, MIME type, description, suffix, and status of all installed plug-ins \|- \| about:cache \| colspan="2" \| Shows information about the Cache Service. \| Translated to "opera:cache" \| \| Translated to "about:konqueror" \|- \| about:mozilla \| colspan="2" \| Shows a page from the fictional ''Book of Mozilla''. In some versions of Netscape, also replaced the meteors in the standard Netscape throbber with a large fire-breathing lizard. \| \| The Windows version of IE showed a blank HTML document with blue background color. Possibly a joke reference to the "Blue Screen of Death". Removed in Windows XP SP2 but it can still be shown by typing "''res://mshtml.dll/about.moz''" instead. \| Translated to "about:konqueror" \|-

Mozilla-specific about: addresses

\|+Mozilla-specific about: addresses ! Addresses ! Mozilla Application Suite/SeaMonkey ! Mozilla Firefox \|- \| about:buildconfig \| colspan="2" \| Shows the arguments and options used to compile the build in use \|- \| about:cache \| colspan="2" \| Shows information about the Cache Service. Number of entries, location of cache, size of cache, etc. for both memory and disk cache. \|- \| about:cache?device=memory \| colspan="2" \| Shows individual entries in memory \|- \| about:cache?device=disk \| colspan="2" \| Shows individual entries on disk \|- \| about:config \| colspan="2" \| Shows an interface for viewing and setting a wide variety of configuration variables, many of which are not otherwise accessible through the GUI (options panels) \|- \| about:credits \| colspan="2" \| Shows a list of all those who contributed to Mozilla \|- \| about:logo \| Shows the logo used on the about: screen \| Not available in Firefox. When entered, a blank page is returned instead of an error page, like with many other about: pages. \|- \| about:license \| colspan="2" \| Shows the Mozilla Public License (and the Netscape Public License) for the piece of software. (Only in products based on Gecko 1.8 or later) \|- \| about:mozilla \| colspan="2" \| Shows a verse from The Book of Mozilla \|- \| about:neterror?e=error&u=url&d=desc \| colspan="2" \| Shows the error page used when the browser could not access the requested path.

! Address ! Flock \|- \| about:myworld \| Shows the My World page. \|-

Opera-specific about: addresses

about: is an alias for opera:, therefore all these addresses also work with about: prefixed. User JavaScript is disabled for all addresses in the about: or opera: URI schemes as a security feature. These pages can, however, be styled using local stylesheets.

None of these functions, except the about:blank combination, work in the Nintendo DS version of the Opera browser, the Nintendo DS Browser.

\|+Opera-specific about: addresses ! Addresses ! Opera \|- \| opera:blank \| Maps to about:blank \|- \| opera:about \| Provides info about the browser and the configured paths. \|- \| about:opera \| Maps to opera:about \|- \| opera:cache \| Shows the content of the cache. \|- \| opera:config \| (Opera 9.0 and newer) Shows a page that allows changing of numerous browser preferences, many of which cannot be accessed through the normal Preferences window \|- \| opera:drives \| Shows the local drives of the host system. \|- \| opera:history \| Shows the content of the browser history. \|- \| opera:plugins \| Shows installed plug-ins. \|- \| opera:button \| Defines a custom button as a set of Opera actions. \|- \| opera:help \| Shortcut to the help directory. This can be either local or external. \|-

Internet Explorer-specific about: addresses

Note: Internet Explorer about: addresses are configurable in Windows. It is therefore possible that some of the listed addresses don't work. For example "about:mozilla" was removed in SP2 (although the page can still be found at "res://mshtml.dll/about.moz"). To configure about: addresses, open the Registry Editor and browse to: "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\AboutURLs". These about: addresses are sometimes used for spyware and adware, most notably in CoolWebSearch, which made about:blank display advertisements.

! Address ! Internet Explorer (SP2) \|- \| about:DesktopItemNavigationFailure \| Displays "navigation cancelled" page. \|- \| about:Home \| Displays the user's home page. \|- \| about:NavigationCanceled \| Displays "navigation cancelled" page. \|- \| about:NavigationFailure \| Displays "navigation cancelled" page. \|- \| about:OfflineInformation \| Informs the user that the current page can not be viewed off line. \|- \| about:PostNotCached \| Informs the user that to refresh the current page, information entered in a form will have to be re-posted. \|- \| about:SecurityRisk \| Informs the user not to browse with the current security settings because they may be harmful to the computer. \|-

In Internet Explorer 7, "about:Tabs" has also been included, and provides information about tabbed browsing. It is loaded when a new tab is created, if the "Open home page for new tabs instead of a blank page" option is selected, which is available under "Internet Options" > "General" tab > "Tabs" section > "Settings" button.

! Address ! Konqueror \|- \| about:konqueror \| Shows a friendly 'start' and navigation page \|-

Epiphany

Since Epiphany uses Gecko as its layout engine, the general and Mozilla-specific about: addresses work as in Mozilla (or Mozilla Firefox, depending on which branch of Gecko is used).

Additionally, about:epiphany yields a quote attributed to Antoine de Saint-Exupéry:

Il semble que la perfection soit atteinte non quand il n'y a plus rien à
ajouter, mais quand il n'y a plus rien à retrancher.

Antoine de Saint-Exupéry

Translated as: ''It seems that perfection is attained not when there remains nothing to add, but when there remains nothing to remove'', this can be seen as a rejoinder to critics who claim that the browser's (and the GNOME desktop's) focus on usability has led to a lack of features.

Netscape (historic)

Older versions of the Netscape browser have an about:people URL that was similar to about:credits above, but it would redirect to Netscape's active employee listing. In addition, about:''username'', where ''username'' is the username of a Netscape employee, would redirect to the Netscape homepage of the employee specified. For example, about:jwz would redirect to http://people.netscape.com/jwz/ (not an active link) http://www.yikes.com/netscape/people.html http://www.jwz.org/gruntle/blowme.html.

Not all employee pages were accessible through this scheme. Only developers that knew which file in the source tree and the encoding scheme used to obfuscate the directory of employee about entries could add their names.

Many other about: easter eggs existed, including the famous about:mozilla, and other less known ones that showed pictures of the Netscape mascot Mozilla in various foreign garb, such as about:deutsche showing Mozilla in Lederhosen.

Some versions of Netscape would display the browser history for the about:global address