liquid-theme-standards

name: liquid-theme-standards description: "CSS, JavaScript, and HTML coding standards for Shopify Liquid themes. Covers BEM naming inside stylesheet tags, design tokens, CSS custom properties, Web Components for themes, defensive CSS, and progressive enhancement. Use when writing CSS/JS/HTML in .liquid files or theme asset files."

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: 1rem;
margin-inline: auto;
border-inline-end: 1px solid var(--color-border);
text-align: start;
inset: 0;

/* Don't: physical properties */
padding-left: 2rem;
text-align: left;
top: 0; right: 0; bottom: 0; left: 0;

Defensive CSS

.component {
  overflow-wrap: break-word;        /* Prevent text overflow */
  min-width: 0;                     /* Allow flex items to shrink */
  max-width: 100%;                  /* Constrain images/media */
  isolation: isolate;               /* Create stacking context */
}

.image-container {
  aspect-ratio: 4 / 3;             /* Prevent layout shift */
  background: var(--color-surface); /* Fallback for missing images */
}

Modern CSS Features

/* Container queries for responsive components */
.product-grid { container-type: inline-size; }
@container (min-width: 400px) {
  .product-card { grid-template-columns: 1fr 1fr; }
}

/* Fluid spacing */
.section { padding: clamp(1rem, 4vw, 3rem); }

/* Intrinsic sizing */
.content { width: min(100%, 800px); }

Performance

• Animate only transform and opacity (never layout properties)
• Use will-change sparingly — remove after animation
• Use contain: content for isolated rendering
• Use dvh instead of vh on mobile

Reduced Motion

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

JavaScript in Liquid Themes

Where JS Lives

Web Component Pattern

class ProductCard extends HTMLElement {
  connectedCallback() {
    this.button = this.querySelector('[data-add-to-cart]');
    this.button?.addEventListener('click', this.#handleClick.bind(this));
  }

  disconnectedCallback() {
    // Clean up event listeners, abort controllers
  }

  async #handleClick(event) {
    event.preventDefault();
    this.button.disabled = true;

    try {
      const formData = new FormData();
      formData.append('id', this.dataset.variantId);
      formData.append('quantity', '1');

      const response = await fetch('/cart/add.js', {
        method: 'POST',
        body: formData
      });

      if (!response.ok) throw new Error('Failed');

      this.dispatchEvent(new CustomEvent('cart:item-added', {
        detail: await response.json(),
        bubbles: true
      }));
    } catch (error) {
      console.error('Add to cart error:', error);
    } finally {
      this.button.disabled = false;
    }
  }
}

customElements.define('product-card', ProductCard);

<product-card data-variant-id="{{ product.selected_or_first_available_variant.id }}">
  <button data-add-to-cart>{{ 'products.add_to_cart' | t }}</button>
</product-card>

JavaScript Rules

AbortController for Fetch

class DataLoader extends HTMLElement {
  #controller = null;

  async load(url) {
    this.#controller?.abort();
    this.#controller = new AbortController();

    try {
      const response = await fetch(url, { signal: this.#controller.signal });
      if (!response.ok) throw new Error(`HTTP ${response.status}`);
      return await response.json();
    } catch (error) {
      if (error.name !== 'AbortError') throw error;
      return null;
    }
  }

  disconnectedCallback() {
    this.#controller?.abort();
  }
}

Component Communication

Parent → Child: Call public methods

this.querySelector('child-component')?.publicMethod(data);

Child → Parent: Dispatch custom events

this.dispatchEvent(new CustomEvent('child:action', {
  detail: { value },
  bubbles: true
}));

HTML Standards

Native Elements First

Progressive Enhancement

{%- comment -%} Works without JS {%- endcomment -%}
<details class="accordion">
  <summary>{{ block.settings.heading }}</summary>
  <div class="accordion__content">
    {{ block.settings.content }}
  </div>
</details>

{%- comment -%} Enhanced with JS {%- endcomment -%}
{% javascript %}
  // Optional: smooth animation, analytics tracking
{% endjavascript %}

Images

{{ image | image_url: width: 800 | image_tag:
  loading: 'lazy',
  alt: image.alt | escape,
  width: image.width,
  height: image.height
}}

• loading="lazy" on all below-fold images
• Always set width and height to prevent layout shift
• Descriptive alt text; empty alt="" for decorative images

JSON Template & Config Files

Theme templates (templates/*.json), section groups (sections/*.json), and config files (config/settings_data.json) are all JSON. Use jq via the bash tool to make surgical edits — it's safer and more reliable than string-based find-and-replace for structured data.

Common patterns

# Add a section to a template
jq '.sections.new_section = {"type": "hero", "settings": {"heading": "Welcome"}}' templates/index.json > /tmp/out && mv /tmp/out templates/index.json

# Update a setting value
jq '.current.sections.header.settings.logo_width = 200' config/settings_data.json > /tmp/out && mv /tmp/out config/settings_data.json

# Reorder sections
jq '.order += ["new_section"]' templates/index.json > /tmp/out && mv /tmp/out templates/index.json

# Remove a section
jq 'del(.sections.old_banner) | .order -= ["old_banner"]' templates/index.json > /tmp/out && mv /tmp/out templates/index.json

# Read a nested value
jq '.sections.header.settings' templates/index.json

Prefer jq over edit for any .json file modification — it validates structure, handles escaping, and avoids whitespace/formatting issues.

References

• CSS patterns and examples
• JavaScript patterns and examples