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