History
The first web browsers were monolithic. Later they adopted a more modular approach and were split into a user interface and an engine.
The engine does most of the work. It essentially takes a URL and a screen rectangle as arguments. It then retrieves the document corresponding to the URL and paints a graphical representation of it on the given rectangle. It handles links, cookies, scripting, plug-ins loading and other matters.
The user interface provides the menu bar, address bar, status bar, bookmark manager, history and preferences window among other things. It embeds the engine and serves as an interface between the user and the engine. Since it provides the graphical elements surrounding the area in which the engine paints documents, the term chrome is sometimes used to refer to it (like the chrome surrounding a car).
The advantage of this modular approach is that it then becomes easy to embed web browser engines in a variety of applications. For example, the same engine used by a web browser can be used by an email client to display HTML mail. On-line help systems integrated in applications have largely moved from using custom formats to using standard HTML displayed with a web browser engine. The EPUB 3 e-book standard is also based on using a layout engine to render XHTML and CSS.
Read more about this topic: Web Browser Engine
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