CSS, JS & HTML Standards for Shopify Liquid Themes

Core Principles

1. Progressive enhancement — semantic HTML first, CSS second, JS third
2. No external dependencies — native browser APIs only for JavaScript
3. Design tokens — never hardcode colors, spacing, or fonts
4. BEM naming — consistent class naming throughout
5. Defensive CSS — handle edge cases gracefully

CSS in Liquid Themes

Where CSS Lives

Critical: {% stylesheet %} does NOT process Liquid. Use inline style attributes for dynamic values:

{%- comment -%} Do: inline variables {%- endcomment -%}
<div
  class="hero"
  style="--bg-color: {{ section.settings.bg_color }}; --padding: {{ section.settings.padding }}px;"
>

{%- comment -%} Don't: Liquid inside stylesheet {%- endcomment -%}
{% stylesheet %}
  .hero { background: {{ section.settings.bg_color }}; } /* Won't work */
{% endstylesheet %}

BEM Naming Convention

.block                      → Component root: .product-card
.block__element             → Child: .product-card__title
.block--modifier            → Variant: .product-card--featured
.block__element--modifier   → Element variant: .product-card__title--large

Rules:

• Hyphens separate words: .product-card, not .productCard
• Single element level only: .block__element, never .block__el1__el2
• Modifier always paired with base class: class="btn btn--primary", never class="btn--primary" alone
• Start new BEM scope when a child could be standalone

<!-- Good: single element level -->
<div class="product-card">
  <h3 class="product-card__title">{{ product.title }}</h3>
  <span class="product-card__button-label">{{ 'add_to_cart' | t }}</span>
</div>

<!-- Good: new BEM scope for standalone component -->
<div class="product-card">
  <button class="button button--primary">
    <span class="button__label">{{ 'add_to_cart' | t }}</span>
  </button>
</div>

Specificity

• Target 0 1 0 (single class) wherever possible
• Maximum 0 4 0 for complex parent-child cases
• Never use IDs as selectors
• Never use !important (comment why if absolutely forced to)
• Avoid element selectors — use classes

CSS Nesting

/* Do: media queries inside selectors */
.header {
  width: 100%;

  @media screen and (min-width: 750px) {
    width: auto;
  }
}

/* Do: state modifiers with & */
.button {
  background: var(--color-primary);

  &:hover { background: var(--color-primary-hover); }
  &:focus-visible { outline: 2px solid var(--color-focus); }
  &[disabled] { opacity: 0.5; }
}

/* Do: parent modifier affecting children (single level) */
.card--featured {
  .card__title { font-size: var(--font-size-xl); }
}

/* Don't: nested beyond first level */
.parent {
  .child {
    .grandchild { } /* Too deep */
  }
}

Design Tokens

Use CSS custom properties for all values — never hardcode colors, spacing, or fonts. Define a consistent scale and reference it everywhere.

Example scale (adapt to your theme's needs):

:root {
  /* Spacing — use a consistent scale */
  --space-2xs: 0.5rem;    --space-xs: 0.75rem;   --space-sm: 1rem;
  --space-md: 1.5rem;     --space-lg: 2rem;       --space-xl: 3rem;

  /* Typography — relative units */
  --font-size-sm: 0.875rem;  --font-size-base: 1rem;
  --font-size-lg: 1.125rem;  --font-size-xl: 1.25rem;  --font-size-2xl: 1.5rem;
}

Key principles:

• Use rem for spacing and typography (respects user font size preferences)
• Name tokens semantically: --space-sm not --space-16
• Define in :root for global tokens, on component root for scoped tokens

CSS Variable Scoping

Global — in :root for theme-wide values Component-scoped — on component root, namespaced:

/* Do: namespaced */
.facets {
  --facets-padding: var(--space-md);
  --facets-z-index: 3;
}

/* Don't: generic names that collide */
.facets {
  --padding: var(--space-md);
  --z-index: 3;
}

Override via inline style for section/block settings:

<section
  class="hero"
  style="
    --hero-bg: {{ section.settings.bg_color }};
    --hero-padding: {{ section.settings.padding }}px;
  "
>

CSS Property Order

1. Layout — position, display, flex-direction, grid-template-columns
2. Box model — width, margin, padding, border
3. Typography — font-family, font-size, line-height, color
4. Visual — background, opacity, border-radius
5. Animation — transition, animation

Logical Properties (RTL Support)

/* Do: logical properties */
padding-inline: 2rem;
padding-block: 1