Slide 1

Client: the user-agent

The user-agent is any tool that acts on the behalf of the user. This role is primarily performed by the Web browser; other possibilities are programs used by engineers and Web developers to debug their applications.

The browser is always the entity initiating the request. It is never the server (though some mechanisms have been added over the years to simulate server-initiated messages).

To present a Web page, the browser sends an original request to fetch the HTML document that represents the page. It then parses this file, making additional requests corresponding to execution scripts, layout information (CSS) to display, and sub-resources contained within the page (usually images and videos). The Web browser then mixes these resources to present to the user a complete document, the Web page. Scripts executed by the browser can fetch more resources in later phases and the browser updates the Web page accordingly.

Slide 2

A Web page is a hypertext document. This means some parts of displayed text are links which can be activated (usually by a click of the mouse) to fetch a new Web page, allowing the user to direct their user-agent and navigate through the Web. The browser translates these directions in HTTP requests, and further interprets the HTTP responses to present the user with a clear response.