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display is CSS's most important property for controlling layout. Every element has a default display value depending on what type of element it is. The default for most elements is usually block or inline. A block element is often called a block-level element. An inline element is always just called an inline element.

block

<div>

div is the standard block-level element. A block-level element starts on a new line and stretches out to the left and right as far as it can. Other common block-level elements are p and form, and new in HTML5 are header, footer, section, and more.

</div>

inline

span is the standard inline element. An inline element can wrap some text inside a paragraph <span> like this </span> without disrupting the flow of that paragraph. The a element is the most common inline element, since you use them for links.

none

Another common display value is none. Some specialized elements such as script use this as their default. It is commonly used with JavaScript to hide and show elements without really deleting and recreating them.

This is different from visibility. Setting display to none will render the page as though the element does not exist. visibility: hidden; will hide the element, but the element will still take up the space it would if it was fully visible.

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other display values

There are plenty of more exotic display values, such as list-item and table. Here is an exhaustive list. We'll discuss inline-block and flex later on.

extra credit

As I mentioned, every element has a default display type. However, you can always override this! Though it wouldn't make sense to make an inline div, you can use this to customize the display of elements that have particular semantics. A common example is making inline li elements for horizontal menus.