CSS, BEM and Context

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One of my favorite CSS architecture topics has been discussed over the last few days: styling things in the context of other things.

It all started with a tweet from Dave Rupert. He asked whether the style .some-context .thing {} belongs into thing.css or some-context.css. Harry Roberts weighed in saying the style should be put in thing.css.

Some days later Harry Roberts wrote a blog article referencing Dave Rupert’s tweet and Jonathan Snook also wrote a blog article referencing both, Dave Rupert’s tweet and Harry’s article (this is getting really meta).

Now what?

So where do we stand taking the input of those three sources? The poll attached to Dave Rupert’s tweet comes to the conclusion, that a slim majority would put .some-context .thing {} in a file named some-context.css. Harry Roberts and Jonathan Snook disagree and argue that thing.css should contain this style.

Both Harry Roberts and Jonathan Snook go a little further and explain different ways how to avoid writing a nested style like that in the first place. Harry Roberts suggests to use a so-called “BEM mix” and Jonathan Snook explains how to use a BEM modifier class to style .thing to avoid styling a specific context but a specific purpose (of “the thing”).

Combining BEM mixes and modifiers

Harry Roberts doesn’t go into much detail about how to use a BEM mix in such a situation (because it is outside of the scope of his article) and Jonathan Snook states, that a BEM modifier would be a better fit for the problem at hand. I’d argue that in many cases a combination of both can be the most beneficial.

So how can we use a combination of BEM mixes and modifiers to handle the styling of a thing in the context of another thing? Let’s build upon Jonathan’s example of a .modal and a .button. First we identify why the button should look different in the context of a modal and we might end up choosing a modifier like .button--primary. But there may still be other styles that are specific to the context (the modal) but need to be on the button. For example we may want to have a lot of whitespace around the button. In situations like those, a BEM mix is the perfect fit.

<div class="modal">
  <h2 class="modal__title">I'm a modal!</div>
  <!-- Other modal content -->
  <button class="modal__button button button--primary">
    Please click me!
// button.css
.button {
  // ... Generic button styles.

.button--primary {
  background: hotpink;
// modal.css
.modal {
  // ... All the modal styles.

.modal__button {
  margin: 3em; // A lot of whitespace.

BEM mix or modifier – where do i put my styles?

You may wonder how to decide which styles do belong in a BEM mix and which styles do belong in a modifier. I’d say that positional styles like margin or top / left are typical candidates for a BEM mix. Other than that font-size, color and text-align may also be potential styles which could be used in a BEM mix under some circumstances. You should ask yourself if a certain style hinders potential reusability outside of a given context, if that is the case, a BEM mix might be the best place to put such a style.


First of all I’d argue that writing a style like .modal .button {} should be avoided by all means. You’ll not only end up wondering where you should put such a style but you’re also opening the pandora’s box of specificity battles.

Try to avoid styling things in a specific context and use modifiers to style for a specific purpose of “the thing” instead. In cases where it isn’t possible to avoid context completely think about using a BEM mix.

Using a BEM mix or a modifier is not a question of either-or they both have specific use cases and often times can be combined symbiotically.

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