Some definitions

The viewport

The viewport is the window or viewing area that displays web pages. When the viewport is smaller than the web page, scroll bars should be available.

The initial containing block

The initial containing block is the entire width and height of your web page - including parts of the page that are outside the viewport.

Containing blocks or containing boxes

A containing block is a box or block that contains other elements (descendant boxes). An element's containing block means "the containing block in which the element lives".

Block-level elements and block boxes

Block level elements (or block boxes) are elements that are formatted visually as blocks. For example, a paragraph of text:

Inline-level elements and inline boxes

Inline elements are elements that do not form new blocks of content; the content is distributed in lines. For example, an emphasized piece of text within a paragraph:

Normal flow

Normal flow is the way a document will display if you had no positioning or floating applied to elements. The content will flow down the page, starting with the first element in your document and finishing with the last element in your document.

Out of normal flow

When a box is taken out of normal flow, all content that is still within normal flow will ignore it completely and not make space for it.

Static positioning

A statically positioned box is one that is in normal flow (see above).

Float positioning

A floated box is positioned within the normal flow, then taken out of the flow and shifted to the left or right as far as possible. Content may flow along the side of a float. More about floats.

Relative positioning

Relatively positioned elements are positioned within the normal flow and then moved. Elements that come after a relatively-positioned element behave as if the relatively-positioned element was still in its ‘normal flow’ position - leaving a gap for it.

Absolute positioning

An absolute positioned box is moved out of the normal flow entirely.

Fixed positioning

Fixed positioned elements are moved out of the normal flow entirely - relative to the viewport. This means that they don't move if the page is scrolled. Win/IE5 and Win/IE6 do not support this positioning method at all.

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Associated with webstandardsgroup.org