Agent Skills: Frontend Conventions

Frontend Conventions

A shared system of rules for naming, formatting, and structuring code across HTML, CSS, and JavaScript. Consistency here is what makes a codebase feel like it was written by one person, regardless of team size.

Code Formatting

General

| Setting | Value | | -------------------------- | ------ | | Indentation | spaces | | Tab size | 2 | | Translate tabs to spaces | Yes | | Trim trailing whitespace | Yes | | New line at end of file | Yes |

Quotes by File Type

| File type | Quote style | | --------- | ----------- | | .html | "double" | | .css | 'single' | | .js | 'single' | | .json | "double" | | .yml | none in general; 'single' when forcing a string or using special characters; "double" when parsing escape codes |

Naming Cases

| What you're naming | Case style | | ------------------------------ | ---------------------------------------------- | | HTML attributes and values | dash-case | | CSS custom property | --dash-case | | JavaScript variable / function | camelCase | | JavaScript class | PascalCase | | File names | dash-case.extension |

Naming Prefixes

All CSS classes are prefixed by category so their purpose is immediately visible in HTML.

| Category | Prefix | | --------------- | ------ | | Component | c- | | Utility | u- | | JavaScript hook | js- |

js- classes are never styled — they exist solely as selectors for JavaScript behavior.

Namespacing

When working within a named project or library, prefix everything with a project namespace to prevent collisions:

| Scope | Namespace | | ----------------- | --------- | | Global/custom property | --namespace-property-name | | Scoped/component property | --prefix-namespace-block-property |

Examples:

/* Global token */
var(--mmn-color-bg-primary)

/* Scoped to a component */
var(--c-mmn-card-border-radius)

Frontend Naming Patterns

| Type | Pattern | Example | | ---- | ------- | ------- | | HTML custom element | <namespace-element-name> | <mmn-card> | | HTML data attribute | data-namespace-attribute | data-mmn-uuid="rng1928" | | CSS selector | class="prefix-namespace-block__element--modifier" | class="c-mmn-card__media--video" | | JS selector | class="js-namespace-target" | class="js-mmn-card-for-ab-test" |

Acceptable Abbreviations

Only use abbreviations from this list — everything else is spelled out in full.

| Full word | Abbreviation | | -------------- | ------------ | | Image | img | | Background | bg | | Minimum | min | | Maximum | max | | Call to action | cta | | Column | col | | Column span | colspan | | Row span | rowspan |

Modifier API

Use these standardized modifier names so any developer can predict class names without checking documentation.

Size Scale

2xs · xs · sm · md · lg · xl · 2xl

<!-- The 2xl heading component -->
<h1 class="c-mmn-heading--2xl">...</h1>

Tints and Shades

xxlight · xlight · light · medium · dark · xdark · xxdark

<!-- The xdark magenta utility color -->
<span class="u-mmn-color-magenta-xdark">...</span>

Hierarchy

primary · secondary · tertiary

<!-- The primary button component -->
<button class="c-mmn-button--primary" type="button">...</button>

Responsive Behavior

Apply a utility or modifier at a specific breakpoint using @from- and @until- suffixes:

| Syntax | Meaning | | ------ | ------- | | behavior@from-[breakpoint] | Apply from this breakpoint and up | | behavior@until-[breakpoint] | Apply below this breakpoint |

<!-- Full width from the sm breakpoint and beyond -->
<div class="u-mmn-full-width@from-sm">...</div>

<!-- Full width below the sm breakpoint only -->
<div class="u-mmn-full-width@until-sm">...</div>

CSS Property Order

Write CSS properties in this order within every rule. Grouping by concern (display → position → size → space → typography → border → background → advanced) makes rules easier to scan and diff.

.selector {
  content;

  /* Display */
  display;
  grid;
  grid-*;
  flex;
  flex-*;
  justify-*;
  align-*;
  place-*;
  order;
  box-sizing;
  appearance;
  visibility;
  opacity;

  /* Position */
  position;
  inset;
  inset-block;
  inset-block-start;
  inset-block-end;
  inset-inline;
  inset-inline-start;
  inset-inline-end;
  float;
  clear;
  transform;
  z-index;

  /* Size */
  inline-size;
  min-inline-size;
  max-inline-size;
  block-size;
  min-block-size;
  max-block-size;
  overflow;
  overflow-block;
  overflow-inline;

  /* Space */
  margin;
  margin-block;
  margin-block-start;
  margin-block-end;
  margin-inline;
  margin-inline-start;
  margin-inline-end;
  padding;
  padding-block;
  padding-block-start;
  padding-block-end;
  padding-inline;
  padding-inline-start;
  padding-inline-end;

  /* Typography */
  font;
  font-family;
  font-style;
  font-size;
  font-weight;
  font-feature-settings;
  font-variant;
  line-height;
  list-style;
  color;
  text-align;
  text-decoration;
  text-transform;
  text-wrap;
  letter-spacing;
  vertical-align;
  cursor;
  pointer-events;
  user-select;

  /* Border */
  border;
  border-radius;
  border-width;
  border-style;
  border-color;
  border-image;
  box-shadow;
  outline;

  /* Background */
  background;
  background-color;
  background-image;
  background-position;
  background-repeat;
  background-size;
  backdrop-filter;

  /* Advanced */
  filter;
  will-change;
  transition;
  animation;
}