The separation of structure, presentation and behavior is dead. It has been dead for a while. Still, this golden rule of web design sticks around. It lives on like Elvis and we need to address it.

I remember the weight of separation vividly. I remember writing custom JavaScript to replace the <a> tag’s target attribute because it added behavior to HTML. I remember dropping the <font> tag from my arsenal of tools, something I don’t regret.

In principle, the rule of separation was good. It helped us explain the best practice and implementation of HTML, CSS and JavaScript. It spun the web toward a brighter future. Separation vanquished the <font> tags, spacer gifs and inline JavaScript that polluted the web for years. In the old web structure, presentation and behavior lived in one layer but the principle of separation divided them.

The old web

The Path

The principle of separation lead web standards into victory during the first browser wars. Separation did not do this by eliminating proprietary standards, but by defining the “path” for new standards like CSS and XHTML. Through this victory browser makers work together now, for the most part. Browsers prefix proprietary CSS and push together for new technologies such as HTML5.

However, separation was an old idea that lived hand in hand with XHTML 2. The idea was that structure, presentation and behavior should be at all times pure. Separation like XHTML 2 has seen its day. Just look at the current state of our specifications. The proof is in the pudding.

The Truth

CSS now has behavior with features like :hover and animations. CSS has structure as well with the pseudo elements ::before and ::after. HTML has the same issues. Its new <input> types have behavior and presentation littered throughout. What the web currently looks like vs what separation wants the web to be is not the same.

The way the web really looks

The specification says separation is dead and the browser makers agree. Last year at SXSW during the browser wars panel I asked the panelists for their thoughts. I asked them what was to keep the <font> tag from coming back and why separation was not being addressed. Brendan Eich answered quickly and simply, “We don’t care about separation.”

He is right to say this. Separation has played its roll. But if he is right and we live in a new age, why do we we still teach separation? Why do we still pretend to follow it? I have a few assumptions.

Why Separation is Still Around

First, it worked at a basic level. It let us know when we were getting into gray areas. Second, the principle was simple and easy to understand. Each technology has its place, HTML, CSS and JavaScript.

So then, what should we do? Ignore all reason and do whatever we like? No, of course not. We need to be honest with ourselves and understand the raising pattern: Divergence.

We have been using Divergence for years. With every site you include :hover in your CSS. With the “email” value you use as your <input> tag’s type. With the many jQuery plug-ins that add new structure and presentation to a site. You use Divergence.


What is Divergence? Divergence is the process you take when any of the three layers of separation cross. Let’s think about what we know the web is like today.


As an example let’s examine a scenario where presentation and behavior cross, such as when you need a design change as the cursor moves over a link. You need to pick CSS or JavaScript.

An example

JavaScript has the mouseover event handler and CSS has :hover. However, you want to add a presentational effect, a nice underline maybe.

During the crossover of the two layers we are forced to choose CSS or JavaScript. You could choose CSS because you want to change the link’s style, but you could also choose JavaScript because the change is triggered by a behavior.

Unlike separation where you are trapped, Divergence says this is fine. In Divergence the lines can cross.

Think of Separation and Divergence as totally different logical approaches.

With Separation you live in a true and false world. Everything is black and white. It is on or off. It is Boolean by nature. With Divergence there’s black and white, but there’s also gray.

Truth is Conditional

The first rule of Divergence is that truth is conditional. Presentation can be behavioral and vice versa. We can use CSS a presentational technology with behavior as in the case of :hover.

However, Divergence does one thing that keeps it from creating a world where anything goes. To understand that concept we need to look at the <font> tag and understand how divergence keeps us from using <font> just as the principle of separation does.

Under the “conditional truth” rule of Divergence the <font> tag is fine. It is presentational and structural. However, with the second rule of Divergence there is an issue. This is the rule of “wholeness”.


The <font> tag is in HTML so it is a structure. However, it is not semantic and so while it is a structure it is not structural. The rule of divergence says that an item who falls into conditional truth is fine. It can be both structural and presentational at the same time. The <font> tag is.

However, the <font> tag must now pass the second rule. This rule, wholeness, says that the item must by nature be compliant wholly… to at least one of its parts. This does not negate the first rule. It only approves the item has the properties of the layer in which it lives.

Wholeness works like this:

  • An item who is in structure must have the properties structure. In other words HTML must be semantic.
  • An item who is in presentation must have the properties presentation. In other words CSS must effect presentation.
  • An item who is in behavior must have the properties behavior. In other words JavaScript must effect behavior.

Divergence says that items can have multiple properties but can only operate from a layer if it belongs to it in wholeness. This means that the item itself must have the properties of the layer it lives in and can demonstrate the properties of another layer if it likes.

Where <font> Fails

The <font> tag meets the conditional truth of Divergence but is not fully structural. It demonstrates the properties of presentation in that you can set “font properties” with it. But, it can not belong to structure because its structural property is not structural. This is because the tag name is purely presentational – “font”. This means that it doesn’t belong fully to at least one of its parts.

Therefore it can not diverge to one side or the other. HTML or CSS. So, we can not use the <font> tag under Divergence.

The principle of Divergence is simple and complex. It apples to all layers of web design.

Some Examples

As a quick example we can say that the HTML5 email value of the <input> tag’s type is fine under Divergence. It is semantic and behavioral but complies wholly to at lest one of its parts HTML.

Simply using the pseudo elements ::before and ::after to create content as structure and not presentation is not divergent because it is not fully presentational. Not to mention inaccessible.

It is unfortunate that the world of web design is not black and white and fortunate at the same time because it moves us forward.

To Round it Up

We are standing in a new age. We need to stop saying separation and doing divergence. In the end we can pretend that the principle of separation is still relevant but it doesn’t support the current direction of the web. It doesn’t help standards and browser makers don’t care.

Most importantly we need practical principles to guide us. So we know when to say yes or no to new features in CSS and HTML so that we can keep one eye on the vendors and the other on the web’s future. We need to accept the fact that separation of structure, presentation and behavior is dead.