CSS Box Model and Display Positioning

5 min readJan 23, 2020

The CSS Box Model

All HTML elements can be considered as boxes. In CSS, the term "box model" is used when talking about design and layout.

The CSS box model is essentially a box that wraps around every HTML element. It consists of: margins, borders, padding, and the actual content. The image below illustrates the box model:

Explanation of the different parts:

Content — The content of the box, where text and images appear
Padding — Clears an area around the content. The padding is transparent
Border — A border that goes around the padding and content
Margin — Clears an area outside the border. The margin is transparent

The box model allows us to add a border around elements, and to define space between elements.

Width and Height of an Element

In order to set the width and height of an element correctly in all browsers, you need to know how the box model works.

Important: When you set the width and height properties of an element with CSS, you just set the width and height of the content area. To calculate the full size of an element, you must also add padding, borders and margins.

Example

This <div> element below will have a total width of 350px:

div {
width: 320px;
padding: 10px;
border: 5px solid gray;
margin: 0;
}

Understanding CSS Positioning

Positioning elements with CSS in web development isn’t as easy as it seems. Things can get quickly complicated as your project gets bigger and without having a good understanding of how CSS deals with aligning HTML elements, you won’t be able to fix your alignment issues.

There are different ways/methods for positioning elements with pure CSS. Using CSS float, display and position properties are the most common methods. In this article, I will be explaining one of the most confusing ways for aligning elements with pure CSS: the position property.

It is worth noting that there are basically two types of ways to display an element in CSS: block and inline.

Display:block;

Block can be, quite literally, seen as a block. It has a specified width and height, optionally controlled by its content, but dimensions set by CSS have prevalence. Another property of elements with display:block is that they do not allow elements on the same “line” as their own. So while an element might have a small width, The next element will always be placed under it. So two simple divs with display block look like this:

Display:inline;

Display:inline is somewhat the opposite. Elements with display:inline always take their width and height from the contents, and will ignore any dimensions specified in the CSS. (Internet Explorer will, incorrectly, let you do this. Thanks NatalieMac!) Another opposite is that, as the name suggest, these elements are displayed in-line, so they will always be next to the preceding element, providing that the preceding element is inline as well and that there is enough width left. If the width of the “line” is filled, an element with display:inline will wrap to the next line. That looks like this:

There is a multitude of other display: properties such as table and inline-block.

Flow

With positioning, the elusive “Flow” of CSS positioning comes into play. The flow in CSS is the logical way in which elements get placed on your screen. For example, when you turn off the CSS on this page, you see the HTML in its original order. That order is the flow, and dictates which element gets rendered and placed where. It’s very simple: the flow of a page is from the top of the HTML to the bottom of it.

The position Property

The position property specifies the type of positioning method used for an element.

There are five different position values:

static
relative
fixed
absolute
sticky

Elements are then positioned using the top, bottom, left, and right properties. However, these properties will not work unless the position property is set first. They also work differently depending on the position value.

Now let’s move on with the position property values…

position: static;

HTML elements are positioned static by default.

Static positioned elements are not affected by the top, bottom, left, and right properties.

An element with position: static; is not positioned in any special way; it is always positioned according to the normal flow of the page.

position: relative;

An element with position: relative; is positioned relative to its normal position.

Setting the top, right, bottom, and left properties of a relatively-positioned element will cause it to be adjusted away from its normal position. Other content will not be adjusted to fit into any gap left by the element.

position: fixed;

An element with position: fixed; is positioned relative to the viewport, which means it always stays in the same place even if the page is scrolled. The top, right, bottom, and left properties are used to position the element.

A fixed element does not leave a gap in the page where it would normally have been located.

position: absolute;

An element with position: absolute; is positioned relative to the nearest positioned ancestor (instead of positioned relative to the viewport, like fixed).

However; if an absolute positioned element has no positioned ancestors, it uses the document body, and moves along with page scrolling.

position: sticky;

An element with position: sticky; is positioned based on the user's scroll position.

A sticky element toggles between relative and fixed, depending on the scroll position. It is positioned relative until a given offset position is met in the viewport - then it "sticks" in place (like position:fixed).