Code Smells in CSS Revisited

Written by on CSS Wizardry.

Way back in 2012, I wrote a post about potential CSS anti-patterns called
Code Smells in CSS. Looking back on
that piece, I still agree with all of it even four years later, but I do have
some new things to add to the list. Again, these aren’t necessarily always bad
things, hence referring to them as code smells: they might be perfectly
acceptable in your use case, but they still smell kinda funny.

Before we start, then, let’s remind ourselves what a Code Smell actually is.
From Wikipedia (emphasis mine):

Code smell, also known as bad smell, in computer programming code, refers to
any symptom in the source code of a program that possibly indicates a deeper
. According to Martin Fowler, ‘a code smell is a surface indication
that usually corresponds to a deeper problem in the system’. Another way to
look at smells is with respect to principles and quality: ‘smells are certain
structures in the code that indicate violation of fundamental design
and negatively impact design quality’. Code smells are usually
not bugs—they are not technically incorrect and do not currently prevent
the program from functioning. Instead, they indicate weaknesses in design
that may be slowing down development
or increasing the risk of bugs or
failures in the future. Bad code smells can be an indicator of factors that
contribute to technical debt. Robert C. Martin calls a list of code smells a
‘value system’ for software craftsmanship.

So they’re not technically, always wrong, they’re just a good litmus test.


Hopefully I can keep this first one nice and brief: I have long been vocal about
the side effects and pitfalls of @extend, and now I would actively consider it
a code smell. It’s not absolutely, always, definitely bad, but it usually is.
Treat it with suspicion.

The problems with @extend are manifold, but to summarise:

  • It’s actually worse for performance than mixins are. Gzip favours
    repetition, so CSS files with greater repetition (i.e. mixins) achieve a
    greater compression delta.
  • It’s greedy. Sass’ @extend will @extend every instance of a class that
    it finds, giving us crazy-long selector chains that look like
  • It moves things around your codebase. Source order is vital in CSS, so
    moving selectors around your project should always be avoided.
  • It obscures the paper-trail. @extend hides a lot of complexity in your
    Sass that you need to unpick more gradually, whereas the multiple class
    approach puts all of the information front-and-center in your markup.

For further reading:

String Concatenation for Classes

Another Sass-based gripe is the use of the & to concatenate strings in your
classes, e.g.:

.foo {
  color: red;

  &-bar {
    font-weight: bold;


Which yields:

.foo {
  color: red;

.foo-bar {
  font-weight: bold;

The obvious benefit of this is its terseness: the fact that we have to write our
foo namespace only once is certainly very DRY.

One less obvious downside, however, is the fact that the string foo-bar now no
longer exists in my source code. Searching my codebase for foo-bar will
return only results in HTML (or compiled CSS if we’ve checked that into our
project). It suddenly became a lot more difficult to locate the source of
.foo-bar’s styles.

I am much more a fan of writing my CSS longhand: on balance, I am far more
likely to look for a class than I am to rename it, so findability wins for me.
If I join a project making heavy use of Sass’ string concatenation, I’m usually
expecting to have a hard time tracking things down.

Of course you could argue that sourcemaps will help us out, or that if I’m
looking for a class of .nav__item then I simply need to open the nav.scss
file, but unfortunately that’s not always going to help. For a little more
detail, I made a screencast about

Background Shorthand

Something else I discussed only recently is the use of background shorthand
syntax. For full details, please refer to the relevant
, but the
summary here is that using something like:

.btn {
  background: #f43059;

…when you probably meant:

.btn {
  background-color: #f43059;

…is another practice I would consider a code smell. When I see the former being
used, it is seldom what the developer actually intended: nearly every time they
really meant the latter. Where the latter only sets or modifies a background
colour, the former will also reset/unset background images, positions,
attachments, etc.

Seeing this in CSS projects immediately warns me that we could end up having
problems with it.

Key Selector Appearing More Than Once

The key selector is the selector that actually gets targeted/styled. It is
often, though not always, the selector just before your opening curly brace
({). In the following CSS:

.foo {}

nav li .bar {}

.promo a,
.promo .btn {}

…the key selectors are:

  • .foo,
  • .bar,
  • a, and
  • .btn respectively.

If I were to take a codebase and ack for
, I might see some output like this:

.btn {}

.header .btn,
.header .btn:hover {}

.sidebar .btn {}

.modal .btn {}

.page aside .btn {}

nav .btn {}

Aside from the fact that a lot of that is just generally pretty poor CSS, the
problem I’m spotting here is that .btn is defined many times. This tells me

  1. there is no Single Source of Truth telling me what buttons look like;
  2. there has been a lot of mutation meaning that the class .btn has many
    different potential outcomes, all via mutable CSS.

As soon as I see CSS like this, I’m aware of the fact that doing any work on
buttons will have a large surface area, tracking down exactly where buttons’
styles come from will be a lot more difficult, and that changing anything will
likely have huge knock-on effects elsewhere. This is one of the key problems
with mutable CSS.

Make use of something like BEM in order to create completely brand new classes
that carry those changes, e.g.:

.btn {}

.btn--large {}

.btn--primary {}

.btn--ghost {}

Just one key selector each.

A Class Appearing in Another Component’s File

On a similar but subtly different theme as above, the appearance of classes in
other components’ files is indicative of a code smell.

Where the previous code smell deals with the question of there being more than
one instance of the same key selector, this code smell deals with where those
selectors might live. Take this question from Dave

If we need to style something differently because of its context, where should
we put that additional CSS?

  1. In the file that styles the thing?
  2. In the file that controls that context?

Let’s say we have the following CSS:

.btn {

.modal .btn {
  font-size: 0.75em;

Where should .modal .btn {} live?

It should live in the .btn file.

We should do our best to group our styles based on the subject (i.e. the key
selector). In this example, the subject is .btn: that’s the thing we actually
care about. .modal is purely a context for .btn, so we aren’t styling it at
all. To this end, we shouldn’t move .btn styling out into another file.

The reason we shouldn’t do this is simply down to collocation: it’s much more
convenient to have the context of all of our buttons in one place. If I want to
get a good overview of all of the button styles in my project, I should expect
to only have to open _components.buttons.scss, and not a dozen other files.

This makes it much easier to move all of the button styles onto a new project,
but more importantly it eases cognitive overhead. I’m sure you’re all familiar
with the feeling of having ten files open in your text editor whilst just trying
to change one small piece of styling. This is something we can avoid.

Group your styles into files based on the subject: if it styles a button, no
matter how it goes about it, we should see it in _components.buttons.scss.

As a simple rule of thumb, ask yourself the question am I styling
x or am I styling y?
If the answer is x,
then your CSS should live in x.css; if the answer is y, it should
live in y.css.

BEM Mixes

Actually, interestingly, I wouldn’t write this CSS at all—I’d use a BEM mix—but
that’s an answer to a different question. Instead of this:

// _components.buttons.scss

.btn {

.modal .btn {

// _components.modal.scss

.modal {

We’d have this:

// _components.buttons.scss

.btn {

// _components.modal.scss

.modal {

  .modal__btn {

This third, brand new class would get applied to the HTML like this:

<div class="modal">
  <button class="btn  modal__btn">Dismiss</button>

This is called a BEM mix, in which we introduce a third brand new class to refer
to a button belonging to a modal. This avoids the question of where things live,
it reduces the specificity by avoiding nesting, and also prevents mutation by
avoiding repeating the .btn class again. Magical.

CSS @import

I would go as far as saying that CSS @import is not just a code smell, it’s an
active bad practice. It poses a huge performance penalty in that it delays the
downloading of CSS (which is a critical asset) until later than necessary. The
(simplified) workflow involved in downloading @imported CSS looks a little

  1. Get the HTML file, which asks for a CSS file;
  2. Get the CSS file, which asks for another CSS file;
  3. Get the last CSS file;
  4. Begin rendering the page.

If we’d got that @import flattened into one single file, the workflow would
look more like this:

  1. Get the HTML file, which asks for a CSS file;
  2. Get the CSS file;
  3. Begin rendering the page.

If we can’t manage to smush all of our CSS into one file (e.g. we’re linking to
Google Fonts), then we should use two <link /> elements in our HTML instead of
@import. While this might feel a little less encapsulated (it would be nicer
to handle all of these dependencies in our CSS files), it’s still much better
for performance:

  1. Get the HTML file, which asks for two CSS files;
  2. Get both CSS files;
  3. Begin rendering the page.

So there we have a few additions to my previous piece on Code Smells. These are
usually all things I have seen and suffered in the wild: hopefully now you can
avoid them as well.

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