BEM: What are modifiers allowed to modify?

Question

Let's say I have the following CSS for a generic list component, using BEM and SCSS:

.list {
  &__item {
    &:not(:last-child) {
      margin-right: .3em;
    }
  }
}

I want to add a modifier that can make the list vertical, like so:

.list--vertical {
  display: flex;
  flex-direction: column;
}

My problem arises when I think about the margin for list__item elements. For vertical lists, I want my margin on the bottom, not right of each item. If I understood BEM correctly, I cannot modify the style of list__item based on the modifer of list, is that correct?

To be more precise, this is what I want to do:

.list--vertical {
  display: flex;
  flex-direction: column;

  .list__item {
    &:not(:last-child) {
      margin-bottom: .3em;
      margin-right: 0;
    }
  }
}

What is the accepted way of dealing with this using BEM? Another modifier for list__item that handles the margin? Another block entirely for vertical lists?

Answer 1

What is the accepted way of dealing with this using BEM?

Depends on what version of BEM you're using. I use a variant of the pre-spec concept of BEM, which means that you'll have different answers if you follow bem.info.

---

Modifiers should be attached to the element they modify. Modifying a block, however, allows the modifier to be inherited by child elements:

<div class="foo foo--example">
  <div class="foo__bar foo--example__bar">...</div>
<div>

This gets messy when child elements have modifiers as well:

<div class="foo foo--example">
  <div class="
    foo__bar
    foo--example__bar
    foo__bar--another-example
    foo--example__bar--another-example">...</div>
<div>

This form of BEM syntax is quite verbose. It's best used on generated code.

---

I use LESS for CSS preprocessing, so my BEM code often looks like:

.foo {
  ...

  &__bar {
    ...
  }
}

With modifiers it becomes:

.foo {
  ...

  &__bar {
    ...
  }

  &--example {
    ...

    &__bar {
      ...
    }
  }
}

This enforces that the cascade is in the proper order and that all selectors continue to have exactly one class.