about URI scheme

about is an internal URI scheme (also known as a "URL scheme" or, erroneously, "protocol") implemented in various Web browsers to reveal internal state and built-in functions. It is an IANA officially registered scheme,[1] and is standardized.

In early versions of Netscape, any URI beginning with about: that wasn't recognized 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 were 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 URI, 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).[2][3]

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. Some browsers use URIs beginning with the name of the browser for similar purposes, and many about URIs will be translated into the appropriate URI if entered. Examples are opera (Opera) or chrome (Google Chrome). An exception is about:blank, which is not translated.

Standardization

In 2010, and onwards, there are efforts to standardize the about URI scheme, and define the processing requirements for some specific URIs, in the IETF Applications Area Working Group (APPSAWG).[4] In August 2012, it was published as an official Request for Comments as RFC 6694. The about URIs that have since been defined and assigned by IANA are listed below.[5]

URI Purpose
about:blank[6] References a blank HTML document with the media type text/html and character encoding UTF-8. This is widely used to load blank pages into browsing contexts, such as iframes within HTML, which may then be modified by scripts.
about:legacy-compat A reserved, though unresolvable, URI defined within HTML5 intended for use in the DOCTYPE designed for compatibility with some legacy authoring tools, such as XSLT, which may not be capable of outputting the more common, shorter alternative that lacks both the PUBLIC and SYSTEM identifiers. <!DOCTYPE html SYSTEM "about:legacy-compat">.
about:srcdoc A reserved, though unresolvable, URI defined within HTML5 intended to be the URI navigated to within iframes whose content comes from the srcdoc attribute.
about:invalid References a non-existent document with a generic error condition. Intended for the case where a URL is necessary, but it should not be resolveable.[7]
about:html-kind Used as an identifier for kinds of media tracks.[8]

Common about: URIs
Common about: URIs
URIs Mozilla / SeaMonkey Mozilla Firefox Google Chrome Chromium Konqueror Opera Safari Internet Explorer / Edge MSN Explorer
about: Shows version information and copyright information Translated to "chrome://version" Translated to "about:konqueror" Translated to "opera:about" Shows a blank page
about:about Lists all the about: URIs
(Firefox since version 4.0)		 Translated to "chrome://about"
Lists all the chrome: URIs
Works like "chrome://chrome-urls"		 Translated to "about:konqueror" Translated to "opera:about" Shows nothing but the text:"about"
about:blank[note 1] Shows a blank document
about:plugins Shows the filename, MIME type, description, suffix, and status of all installed plug-ins Deprecated in Chrome 57[9] Shows the filename, MIME type, description, suffix, and status of all installed plug-ins Translated to "opera:plugins"
about:cache Shows information about the Cache Service. Deprecated in Chrome 66[10] Translated to "about:konqueror" Translated to "opera:cache"
about:mozilla 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. Doesn't work (unresolvable) Translated to "about:konqueror" 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.

Mozilla-specific about: URIs

Many of these can also be used in Thunderbird, by setting them as the "Mail Start Page". Also, some extensions define additional about: URIs not listed here.[11]

Mozilla-specific about: URIs
URIs Mozilla Application Suite / SeaMonkey Mozilla Firefox
about:addons Shows installed addons, plugins, themes, and provides an interface for installing new addons. Firefox 4+, SeaMonkey 2.1+
about:app-manager Shows the Firefox OS Simulator.
about:bloat Displays BloatView output (disabled in release builds). Superseded by about:memory on Gecko 1.9.2.[12]
about:bloat?new TBA
about:bloat?clear TBA
about:blocked Shows the malware protection page used when the browser identifies a page as not safe for viewing. Firefox 3+
about:buildconfig Shows the arguments and options used to compile the build in use.
about:cache 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 Shows individual entries in memory.
about:cache?device=disk Shows individual entries on disk.
about:cache?device=offline Shows individual entries for offline viewing. Gecko 1.9+
about:cache-entry Shows information about a cache entry. Used in about:cache links. Requires parameters.
about:certerror Shows the error page used when an SSL/TLS certificate is untrusted or otherwise invalid.
about:config 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).

The known earliest use of about:config dates back to Netscape 4.x, where it was not possible to edit the settings from the browser.

about:config settings first became modifiable in Mozilla Application Suite.

On first use, about:config displays a message which tells the user that the settings might void the warranty and they can be "Harmful to the stability, security, and performance of this application." The message about voiding one's warranty is a joke, as Mozilla Firefox ships without a warranty of any kind.[13] The message has a checkbox to turn it off.

about:crashes Shows details of crashes submitted by Mozilla's crash reporter. Firefox 3+, SeaMonkey 2.0+
about:credits Shows a list of all those who contributed to Mozilla.
about:devtools Shows the welcome page used by Firefox Developer Tools.
about:downloads Shows a list of downloaded files.
about:feeds Shows the page used when clicking the rss feed icon in the address bar.
about:healthreport Shows the Firefox Health Report.
about:home Shows the Firefox 4 default home page.
about:jetpack Shows an interface for viewing and setting the JetPack extension.
about:licence In the British English localised version, shows the content of about:license but with all occurrences of license used as a noun replaced with licence (see American and British English spelling differences). Not available in Firefox 4
about:license Shows the Mozilla Public License (and the Netscape Public License) for the piece of software. Gecko 1.8+
about:logo Shows the logo used on the about: screen.
about:memory Shows memory usage. Firefox 3.6+
about:mozilla Easter egg, shows a verse from The Book of Mozilla.
about:neterror Shows the error page used when the browser could not access the requested path.
about:newaddon This dialog is shown when one or more add-ons have been added without using the normal Add-on Manager procedures, by dropping it in one of the "special" add-on directories. It asks which ones to keep enabled.

Typing about:newaddon in the URL bar won't display anything. (It may briefly flash an empty error dialog, then the empty about:blank page is displayed.)

about:newtab Shows a grid of favourite and most-visited websites, Firefox 13+.
about:permissions Shows permissions for all sites on Firefox 6.0a1.
about:plugins Shows installed, currently enabled plug-ins.
about:preferences Shows the new preferences page.
about:privatebrowsing Can be used to switch to private browsing mode. It shows a message indicating that Firefox will not remember any history for the current session. Note: It does not show URI on the address bar.
about:reader?url= Opens the specified URL in Reader Mode. For example, about:reader?url=http://example.com.
about:rights Shows licensing information.
about:robots Easter egg, beginning with March 8, 2008 trunk builds. The title of a window contains a citation from The Day the Earth Stood Still.[14]
about:searchreset TBA
about:sessionrestore Shows an interface for viewing about last session.
about:socialerror TBA
about:startpage In Ubuntu, shows the Ubuntu start page with the current search engine (if the addon Ubuntu Firefox Modifications – ubufox is installed).
about:support Shows troubleshooting information. Firefox 3.6+, SeaMonkey 2.1+
about:sync-tabs Shows list of tabs from other computers. Requires enabled synchronization.
about:tabcrashed TBA. Firefox 25+
about:webrtc Shows an interface for WebRTC internals.
about:welcome Shows the welcome page used by Firefox.

Opera (Presto)-specific about: URIs

about is an alias for opera; therefore all these URIs also work with about prefixed. User JavaScript is enabled for all URIs 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 opera:about combination, work in the Nintendo DS version of the Opera browser, the Nintendo DS Browser.

Opera-specific about: URIs
URIs 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 (Operating system specific) Shows the local drives of the host system.
opera:history Shows the content of the browser history.
opera:historysearch (Opera 9.5 and newer) Shows the start page of internal search engine for the browser history.
opera:plugins Shows installed plug-ins. Opera 11 and newer allow individual plug-ins to be disabled from this page.
opera:button Defines a custom button as a set of Opera actions. (Note: This does not work directly, all button parameters as on CustomButtons need be specified.)
opera:help Shortcut to the help directory. This can be either local or external.
opera:debug (Opera 9.5 and newer) Shows a page that allows changing of settings for remote debug of Opera via Opera Dragonfly
opera:memdebug (Opera 9.5 and newer) Shows debug information about memory usage.
opera:webstorage (Opera 10.5 and newer) Shows information Web Storage (see HTML5).
opera:webdatabases (Opera 10.5 and newer) Shows information Web Storage databases (see HTML5).
opera:gpu Shows information about hardware accelerated graphics and GPU support.
opera:cpu (Introduced in Opera 12 beta). Displays CPU usage.

Internet Explorer-specific about: URIs

Internet Explorer about URIs are configurable in Windows. It is therefore possible that some of the listed URIs will not work on a particular computer. For example, "about:mozilla" was removed in SP2 (although the page can still be found at "res://mshtml.dll/about.moz"). These about URIs are sometimes used for spyware and adware, most notably in CoolWebSearch, which made about:blank display advertisements.

Note: Any about URI that is not recognized by Internet Explorer redirects to a page saying "Navigation to the webpage was canceled"

In more recent versions of Internet Explorer, many of the following do nothing, or have since been removed.
URI Internet Explorer 6 (SP2) Internet Explorer 7 Internet Explorer 8 Internet Explorer 9-10 Internet Explorer 11
about:home Displays the user's home page. Not supported. Displays the user's home page.
about:blank Displays the blank page.
about:desktopitemnavigationfailure Displays "navigation cancelled" page. No longer supported.
about:navigationcanceled Displays "navigation cancelled" page. No longer supported.
about:navigationfailure Displays "navigation cancelled" page. No longer supported.
about:noadd-ons Displays information page when add-ons are disabled.
about:noadd-onsinfo Displays information page about the effects when add-ons are used.
about:offlineinformation Informs the user that the current page can not be viewed off line. No longer supported.
about:postnotcached Informs the user that the current page needs to refresh and any information entered in a form will have to be re-posted. The page has the title: "Webpage has expired"
about:securityrisk Informs the user not to browse with the current security settings because they may be harmful to the computer.
about:tabs Informs the user about tabbed browsing. It is also loaded when a new tab is created if the "Open home page for new tabs instead of a blank page" setting is enabled.
about:inprivate Informs the user about InPrivate Browsing. It is loaded when InPrivate Browsing is initiated.
about:compat Displays a table of sites and the document rendering emulation mode configured for maximum backward compatibility.[15]
about:newsfeed Displays a page analogous to the "Top sites and suggested content" in the Edge browser with a Bing search bar at the top, followed by a customizable (even if not signed in with a Microsoft Account) row of website tiles (equivalent to the Favorites Bar functionality), followed by a row of news topic category selection links and an infinitely scrolling news aggregation feed.

Edge-specific about: URIs
URI Microsoft Edge
about:home Shows the content that would be shown if the home button were pressed
about:flags Displays a list of Developer Setting and Experimental features
about:start Shows the customizable (default) start page.
about:config Redirects to about:flags
about:tabs Shows the (default) new tab page.
about:compat Shows the Enterprise Mode list (if configured)
about:edge Shows the Edge logo.
about:inprivate Shows the inprivate information tab.

Note: Any "about:" URI is accepted, but only returns a page with the message "Well, that didn’t work".

Chrome-specific about: URIs
URI[16] Google Chrome
about:cache Deprecated in version 66[10]
about:crash Crashes the current rendering process. (This causes the "Aw, snap!" crash notification page to be displayed.)
about:credits Displays the list of free and open source software used in the browser, and their associated licenses.
about:dino Indicated that there are no connection.
about:dns Displays the DNS records
about:flags See about:labs
about:gpu Displays information about WebGL and hardware accelerated graphics.
about:histograms Displays histograms
about:inducebrowsercrashforrealz Crashes the Google Chrome browser. This is designed for developers to test what happens when the Google Chrome browser crashes.
about:internets On particular versions of Windows, displays a page entitled "Don't Clog the Tubes!" which renders a page with an animation of the Microsoft Windows "3D Pipes" screensaver. Does not work on Windows Vista due to the pipes screen saver no longer being supported for releases after the 2.0.169.1 release.
about:kill Same as about:crash, but with a violet background color and the message "He's Dead, Jim!" Occurs when the OS runs out of memory or the browser is killed by an external cause, such as the OS shutting down.
about:labs Displays a page for testing features. Moved to about:flags in Chrome Dev channel 8.0.552.11
about:memory Displays the process manager
about:net-internals Provides an interface for monitoring the network usage and performance statistics
about:plugins Shows installed plug-ins. Deprecated in Chrome 57[9]
about:sandbox Shows which sandbox protection mechanisms are currently enabled.
about:shorthang Hangs the tab's process, making it become unresponsive. This is designed for testing the hanging protection.
about:stats Displays statistics about processes. At the top of the page, it states "Shhh! This page is secret!"
about:version Displays version information, same as about:

Note: the above list is not exhaustive; for a full list, see about:about URI.

Note: "about:" is usually (with the exception of e.g. "about:blank") translated to a "chrome://" URL in Chromium and newer versions of Google Chrome.

Konqueror

Note: Any about URI except about:blank and about:plugins redirects to about:konqueror.

URI Konqueror
about:konqueror Shows a friendly ‘start’ and navigation page

GNOME Web

Note: When GNOME Web (Epiphany) used Gecko as its layout engine, the general and Mozilla-specific about: URIs worked as in Mozilla Firefox. Since WebKit became Epiphany's layout engine, only these URIs are supported:

URI GNOME Web
about:blank Displays the blank page.
about:plugins Shows installed plug-ins.
about:memory Shows details about memory usage.
about:epiphany Displays the following message:[17]

   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.[note 2]   

— Antoine de Saint-Exupéry   

It is a quote from Chapter 3 of Saint-Exupéry's memoir Terre des Hommes (titled Wind, Sand and Stars in its English translation by Lewis Galantiere).

about:applications Interface for managing web applications, supported since release 3.2.[18]
about:overview Displays a grid of thumbnails for the more visited websites.
about:gpu Displays information about WebGL and hardware accelerated graphics.
about:web Displays the browser and its engine information, same as about:

Netscape (historical)

The about: URL originated in and has existed in all versions of Netscape browsers. It was originally added as an Easter egg to display information about the development team.[19]

Older versions of the Netscape browser have an about:people URI 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).[20] Not all employee pages were accessible through this scheme. Only developers who 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:deutsch showing Mozilla in Lederhosen.[21]

Some versions of Netscape would display the browser history for the about:global URI.[21]

Internet Explorer for Mac OS 5 (historical)

In Internet Explorer for Mac 5 was an offline Easter egg, accessible by typing ‘about:tasman’, showing the Acid1 test with the text replaced by the names of the developers.[22]

Safari

Only about:blank is accepted; all other about: URIs trigger default address bar behavior (e.g. a Google search).

Microsoft Outlook

In Microsoft Outlook, an additional outlook:today URI was added, which showed the Outlook Today Screen (it shows messages, tasks and appointments and is also accessible from IE). Also, although about:blank is supported, anything else (including about:about) typed after about: would show that text as a web page in the browser.

Notes
  1. The term "about:blank" is also the generic name for a group of variants of the CoolWebSearch malware program group.
  2. It seems that perfection is attained not when there remains nothing to add, but when there remains nothing to remove.

References
  1. "Uniform Resource Identifier (URI) Schemes". www.iana.org. Retrieved 2016-09-28.
  2. Dominik Hoffmann (1996-06-14). "Special Netscape URLs for cache diagnostics?". Newsgroup: comp.infosystems.www.browsers.mac. Retrieved 2012-09-29.
  3. Matthew Paul Thomas (2000-06-23). "Weird easter egg in NN 3.0 or 3.01". Retrieved 2012-09-29.
  4. "Appsawg Status Pages". IETF.
  5. ""about" URI Tokens". www.iana.org. Retrieved 2017-06-28.
  6. RFC 6694
  7. "CSS Values and Units Module Level 3". www.w3.org. Retrieved 2017-06-28.
  8. "HTML Standard".
  9. "615738 - chromium - An open-source project to help move the web forward. - Monorail". bugs.chromium.org. Retrieved 2020-01-01.
  10. "811956 - chromium - An open-source project to help move the web forward. - Monorail". bugs.chromium.org. Retrieved 2020-01-01.
  11. "Debugging memory leaks". Mozilla Developer Network.
  12. "446704 - "This might void your warranty!" is not appreciated by corporate customers". Bugzilla. Mozilla. 2008-07-22. Retrieved 2014-07-07.
  13. "Bug 417302 — about:robots". Mozilla Project Bugzilla.
  14. "Enterprise Mode and the Enterprise Mode Site List (Internet Explorer 11 for IT Pros)". Docs.microsoft.com. Retrieved 2018-06-19.
  15. "Screenshot of Epiphany 3.4".
  16. "NEWS file". The GNOME Project's git repository. Retrieved 2011-10-17.
  17. "A short history of the about: URL".
  18. "About: Netscape People". Yikes!. Archived from the original on 2006-05-20. Retrieved 2006-04-21.
  19. "This page is dedicated to Netscape Navigator 3.0b4 and followers, oops Mozilla 3.0b4 and friends!".
  20. "Internet Explorer Easter Egg — IE5 Mac Team". The Easter Egg Archive. 2000-06-19.
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