Microsoft’s proprietary browser engine for Internet Explorer was Trident, also known as MSHTML.
Trident first appeared with Internet Explorer 4.0 in 1997 and remained the central rendering engine through Internet Explorer 11. A browser engine interprets web standards and turns HTML, CSS, and scripts into the page users see. Trident was deeply tied to Windows and to the compatibility requirements of Microsoft’s long-running browser platform.
The name MSHTML is especially common in Microsoft documentation and software components, while Trident is the familiar browser-engine label. Trident is not the same as EdgeHTML, which Microsoft created later for the original Microsoft Edge. It is also distinct from Gecko, used by Firefox; WebKit, associated with Safari and historically other browsers; and Presto, used by older Opera releases.
Internet Explorer’s legacy engine still mattered after Edge launched because Microsoft Edge’s IE mode uses the Trident/MSHTML engine for sites that depend on older Internet Explorer technologies.