Web Dev Reference
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3-tier application a program that is organized into three major parts: the workstation or presentation interface; the business logic; and the database and related programming. Each of these is distributed to one or more separate places on a network. agile software development calls for keeping code simple, testing often, and delivering small, functional bits of the application as soon as they're ready. The focus is to build a succession of parts, rather than delivering one large application at the end of the project.
It is useful to make a distinction between the vocabulary of an HTML document—the elements and attributes, and their meanings—and the syntax in which it is written.
HTML Element Content Models An HTML element's content model specifies what is allowed inside the element.