Short for MIME Encapsulation of Aggregate Documents, such as HTML, MHTML is a file format that stores the entire contents of a web page in a single file (.mht or .mhtml). For example, this HTML (hypertext markup language) web page, its CSS (cascading style sheets), and images could be saved in a single file called "example.mht," and easily shared with and opened by other people.
The MHTML file format was proposed in RFC 2557 in March 1999 and was first available with Internet Explorer 5.0. Since then, it's been natively supported in versions of Chrome, Edge, Opera, Vivaldi, and Yandex. Other browsers, like Firefox and Safari, that do not natively support MHTML can use an add-on for MHTML support.