multipart/related. The first of the file is normally encoded HTML; subsequent parts are additional resources identified by their original URLs and encoded in base64. This format is sometimes referred to as MHT, after the suffix .mht given to such files by default when created by Microsoft Word, Internet Explorer or Opera. MHTML is a proposed standard, circulated in a revised edition in 1999 as RFC 2557.
The .mht format was introduced in 1999 with Internet Explorer 5. Saving in this format allows users to save a web page and its resources as a single MHTML file called a "Web Archive", where all images and linked files will be saved as a single entity. It may, however, be unable to save certain complex web pages correctly, especially those containing scripts.
Support for saving a web page with its resources as an MHTML file was made available in the Opera 9.0 web browser since the weekly build 8265 (released on March 10, 2006). In Opera 9.50 the default saving format for "Save as" function is MHTML. In Opera 9.50 it is possible to save web pages in following formats: 1) only with HTML, 2) HTML with pictures saved in separate folders 3) in TXT format and 4) with MHTML format.
Mozilla Firefox 3 and earlier versions do not include support for saving or opening web pages as MHTML files; there is, however, source code available for viewing MHTML files within the related Thunderbird project. This is filed as unsolved bug 18764 within the Firefox project since 1999.
There is an extension which adds MHTML support: UnMHT This works in all versions of Firefox from 1.0 to the current one (as of 3.0.1). It is available under the MPL 1.1/GPL 2.0/LGPL 2.1 licence. The current version (v4.7.4) can write and open MHT files. It includes English, German and Spanish locales. MHT files exported from MS Office (e.g. PowerPoint) are not supported.
For Firefox 1.5, this functionality can be obtained on Windows and Linux operating systems by installing a freely available third-party XPI file from Mozilla Archive Format extension, though the .mht files it generates are not fully compatible with Microsoft's products.
As of version 3.1.1 (released 2008-05-28), Apple Inc.'s Safari web browser does not support the MHTML format. Instead, it saves web pages in the webarchive format, which were directory bundles pre-3.0, but are now single file outputs.
There is a UnMHT port for Safari available based on SIMBL and GreaseKit.
As of version 3.5.7 (released 2007-05-02), KDE's Konqueror web browser does not support MHTML files. However, it does include a feature for saving web pages as single files ("web archives", file extension .war) which are actually gzipped tarballs. (but there is a side project mhtconv that allows to use MHTML with Konqueror for both 3.x and 4.x KDE branches)
NetFront 3.4 (on devices such as the Sony Ericsson K850) can view MHTML files, and also save open pages as mhtml (created with filename suffix .mhtml).
MHTML is also supported by Microsoft Word Viewer 2003, Microsoft Office Access 2003, Excel 2003, OneNote 2003, and PowerPoint 2003. Surprisingly, current versions of Microsoft's mainstream HTML editor products, such as Microsoft FrontPage and Microsoft Expression Web are unable to understand and parse MHTML format. They can only be forced to open MHTML files and display their contents as simple text. Microsoft SharePoint Designer 2007 has been reported to handle MHT files correctly.