The webarchive file format is available on Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 and Windows for saving and reviewing complete web pages using the Safari browser

Safari (web browser)

Safari is a web browser developed by Apple Inc. and included with the Mac OS X and iOS operating systems. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Safari is also the...

. Support for webarchive documents was added in Safari 4 Beta on Windows; Safari 3 on Windows does not support the format.

The webarchive format is a concatenation of source files with filenames saved in the binary plist

Property list

In the Mac OS X, iOS, NeXTSTEP, and GNUstep programming frameworks, property list files are files that store serialized objects. Property list files use the filename extension .plist, and thus are often referred to as p-list files....

 format using NSKeyedEncoder. Although it is a platform-independent format, many people prefer to use Safari's Print to PDF

Portable Document Format

Portable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....

 feature instead to store webpages. Indeed, the .webarchive format appears to more be a convenience for Mac developers. The API uses webarchives to simplify using cutting-and-pasting with whole or partial web pages.

Converting for other Browsers

Workarounds to allow the file to be viewed in other browsers are possible, though specific webpage contents may hinder this process:

• Open the webarchive file using Microsoft Word
  Microsoft Word
  Microsoft Word is a word processor designed by Microsoft. It was first released in 1983 under the name Multi-Tool Word for Xenix systems. Subsequent versions were later written for several other platforms including IBM PCs running DOS , the Apple Macintosh , the AT&T Unix PC , Atari ST , SCO UNIX,...
  
   and save it as a web page from Word's file menu.
• Use the free tools WebArchive Folderizer (for OS X 10.2 and higher) or WebArchive Extractor (for OS X 10.4.3 and higher).

Alternatives

Other web browsers use the MHTML

MHTML

MHTML, short for MIME HTML, is a web page archive format used to combine resources that are typically represented by external links together with HTML code into a single file. The content of an MHTML file is encoded as if it were an HTML e-mail message, using the MIME type multipart/related...

 format or do the equivalent by saving a directory of inline resources (usually images) alongside the html file, sometimes compressed, like the .war format used by Konqueror

Konqueror

Not to be confused with the Conqueror web browser.Konqueror is a web browser and file manager that provides file-viewer functionality for file systems such as local files, files on a remote ftp server and files in a disk image. It is a core part of the KDE desktop environment...

(tar+gzip or tar+bzip2). Safari isn't able to handle either format.