An MHTML document is an archive document type that combines a web page and all of its resources. This is significantly different from HTML, which only holds the text of a web page. While HTML is simply a markup language, MHTML is a complete file format. Since an MHTML document is one file, rather than one file plus a folder of resources, it is often much easier to handle than simple HTML documents.
A standard was created in 1999 that determines how an MHTML document should be structured. All documents of this type use the standard to some degree. There are many varieties of MHTML documents that come from this standard. The differences between the document types does not come from the way the documents are structured, but from how they are saved. Although the format of a document is standardized, the method by which a web page and its resources are processed into such a document is not.
Differences in document processing can cause compatibility issues when an MHTML document is opened in a web browser. Some document formats can be opened reasonably well in different browsers, but others have a proprietary format that can only be opened in the browser that created them. As a rule, it is usually better to open these documents in the same browser in which they were created, because that gives the most faithful recreation of the original web page. Opening a document in a different browser doesn't usually cause significant layout difficulties, although the possibility exists. In addition, some browsers do not support MHTML documents without plugins or add-ons.
The version of a web page held in these documents are static, meaning they do not have the capability to dynamically update with a web page. Viewing an MHTML document is similar to viewing an HTML document, in that both documents are snapshots of a web page at a particular point in time. In order to create an updated document, the page must be saved again at a new time.
MHTML documents are commonly used to send large web pages over email or make the transfer of smaller web pages easier. Since it is not possible to easily attach a folder of resources to an email, the MHTML document archive format is a quick and easy way around this problem. Sending these documents through email has a wide variety of applications, most notably in web publishing and collaboration.