How to understand CSS positioning?

The CSS Positioning property allows you to position elements.

CSS positioning and floating

CSS provides some properties for positioning and floating. Using these properties, you can create a column layout and combine part of the layout with The other part overlaps and can also accomplish tasks that have often required the use of multiple forms over the years. (Recommended study: CSS Introduction Tutorial)

The basic idea of ​​positioning is simple. It allows you to define where the element box should appear relative to its normal position, or relative to its parent. The position of an element, another element, or even the browser window itself.

Obviously, this function is very powerful and surprising. It shouldn't be surprising to know that user agents support positioning in CSS2 much better than other aspects.

Floats, on the other hand, were first proposed in CSS1 and were based on a feature added by Netscape in the early days of the Web. Floating isn't exactly positioning, but it's certainly not a normal flow layout either. We will clarify the meaning of float in a later chapter.

Everything is a box

A div, h1 or p element is often called a block-level element. This means that these elements appear as a block of content, a "block box". In contrast, elements such as span and strong are called "inline elements" because their content appears within a line, an "inline box."

You can change the type of generated box using the display property. This means that you can make inline elements (such as elements) behave like block-level elements by setting the display property to block.

You can also set display to none so that the generated element has no frame at all. This way, the box and all its contents are no longer visible and take up no space in the document.

But in one case, block-level elements are created even without explicit definition. This happens when you add some text to the beginning of a block-level element (such as a div). Even if the text is not defined as a paragraph, it will be treated as one:

<div>
some text
<p>Some more text.</p>
</div>

In this case, the box is called an unnamed block box because it is not associated with a specifically defined element.

A similar situation occurs with text lines of block-level elements. Suppose you have a paragraph containing three lines of text. Each line of text forms an unnamed box. You cannot directly apply styles to nameless blocks or line boxes because there is no place to apply styles (note that line boxes and inline boxes are two different concepts). However, it helps to understand that everything you see on the screen forms some kind of box.

CSS positioning mechanism

CSS has three basic positioning mechanisms: normal flow, floating and absolute positioning.

All boxes are positioned in the normal flow unless specifically specified. That is, the position of an element in the normal flow is determined by the element's position in (X)HTML.

Block-level boxes are arranged one after another from top to bottom, and the vertical distance between boxes is calculated from the vertical margin of the box.

Inline boxes are arranged horizontally in a row. Their spacing can be adjusted using horizontal padding, borders, and margins. However, vertical padding, borders, and margins do not affect the height of the inline box. The horizontal box formed by a line is called a line box. The height of a line box is always high enough to accommodate all the inline boxes it contains. However, setting the row height can increase the height of this box.

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