I recently came across a discussion about functional CSS, and it made me think about separation of concerns. See, I realized that both sides of the conflict are equally right: those who propogate for functional CSS and those who prefer the traditional method use the same argument.

In order to explain, I will first try to clarify what I believe each side’s argument to be.

1. Functional CSS

If we begin with functional CSS, I believe that its ultimate goal is this:

Your CSS should be reusable, and your HTML replaceable; if you create a good-looking stylesheet for one page, you should be able to use it for another page without having to modify the CSS.

The consequence of this argument is that all CSS should be strictly presentational and not depend on any specific semantic organization of the HTML.

For example, if you wanted to specify a style for buttons, you would create a CSS rule for .button:

.button { /* looks like a button */ }

Very reasonable! Now, you just have to add that class to everything in your HTML structure that should be styled as a button:

<a href="login" class="button">Log in</a>

With the popularity of CSS frameworks like Bootstrap, it is obvious that this discipline has met much success and won many hearts. If there are a dozen different CSS stylesheets that define .button styles, you could change the appearance of your HTML page by simply switching out the stylesheet.

2. Traditional discipline

Now, let’s explore the argument against functional CSS:

Your HTML should be reusable, and your CSS replaceable; if you create an HTML document, you should be able to change its appearance without having to modify the HTML.

As you can see, this is the exact same argument, except it’s been turned on its head. The consequence of this argument is that all HTML should be strictly semantic and not depend on any specific stylesheet.

For example, the aforementioned login button should rather be given a semantic id:

<a href="login" id="login">Log in</a>

And to define its appearance, you should create a CSS rule for #login rather than for .button:

#login { /* looks like a button */ }

This earns us the benefit of not having to change the semantic structure of our document whenever we might want to change its presentation. If somebody decides that our login link should look not like a button, but like a normal link, he or she needn’t modify the HTML to achieve this. This is reasonable: because the appearance of the link is a presentational issue, it belongs in the CSS, not in the HTML.

3. A comparison

The traditinal discipline aligns nicely with the role of HTML as a semantic definition of a document – but on the other hand, the functional CSS perspective respects the role of CSS as a presentational description of a document.

As for “separation of concerns”, we must admit it to be the cornerstone of both disciplines; they just approach it from different angles, and in doing so, each appears to miss the other’s point:

In the traditional method, while presentational issues are kept out of the semantic organization of the HTML, the semantics are bound to infect the presentational matters of the CSS. And while functional CSS keeps semantics out of the presentational description, it fills the semantic description with tons of presentational concerns.

The result is that neither discipline enforces separation of concerns.

4. A solution

If genuine separation of concerns is what we truly desire – that is, a strict dividing line between semantics and presentation – then I think I have a solution:

Presentational description:

@mixin button { /* looks like a button */ }

Semantic description:

<a href="login" id="login">Log in</a>

Semantic–presentational link:

#login { @include button }

In text:

  1. Forbid semantic1 information in the presentational description.
  2. Forbid presentational information in the semantic description.
  3. Create a linking stylesheet that defines the connection between the presentation and the semantics.

This way, you can replace either part, as long as you modify the linking stylesheet accordingly.

I have started to use this discipline – I guess we can call it “extreme separation of concerns” – for my own website, and my own experience is that it works relatively well.


  1. Note that I use the word “semantic” here to refer to the structure of the HTML. I don’t mean that the presentational stylesheet mustn’t describe elements like a or code – these selectors have very little to do with the document’s specific structure and are likely to be used identically in all HTML documents. I mean that it shouldn’t describe things like #login, .post-meta time or #header .subtitle.

    Personally, I do try to avoid even a and code in my presentational description, but this isn’t necessary for the discipline to work.