Docs :: Getting Started

The file structure is a simplified ITCSS: files are organized in layers going from broad and generic (tokens, element defaults) to specific and local (components, one-off helpers). When done right, it takes care of all the common “shortcomings” of CSS such as specificity wars and cascading conflicts.

mCSS implements this with native CSS cascade layers: every framework file is imported into a named @layer, and the layer order (not the import order or specificity) decides who wins. Anything you write outside the layers beats the framework by default, so mCSS always gets out of your way. The only exception is helper classes, which use !important so they can override anything, including your own CSS.

Two folders

The styles live in two folders, and the split is the methodology in miniature:

• framework/: mCSS itself, every file in a named layer. You copy it and mostly leave it alone: customization goes through the settings.* tokens, a theme, or your own CSS, not through edits scattered across framework files.
• site/: this site’s own CSS, imported unlayered, after the framework, so it wins without specificity games. Because those files are unlayered, the order you import them in is important so make sure you have a clear mental model of ITCSS principles before delving in. You can also refer to mCSS own /site’s setup as a reference.

The layers, in order

Each layer only ever loses to the ones after it. The sections below detail each one; most have full docs in the sidebar.

The framework’s own entry, framework/mcss.css, makes the split concrete: it declares the layer order and puts every core framework file in its layer. You never edit it:

/* framework/mcss.css (abridged) */
@layer settings, base, elements, global, components, theme, helpers;

/* Build-time only (media queries + mixins definitions) */
@import url(./settings.media-queries.css);
@import url(./settings.mixins.css);

@import url(./settings.tokens.css) layer(settings);
@import url(./settings.ui.css) layer(settings);
@import url(./base.reset.css) layer(base);
@import url(./elements.text.css) layer(elements);
@import url(./global.grid.css) layer(global);
@import url(./help.spacing.css) layer(helpers);
/* etc. — note: no component.* and no theme.* imports here */

Two layers are declared but never filled by this entry. The theme slot is yours (see below), and the components slot belongs to the component library, which lives in its own entry, framework/mcss.components.css. Import it alongside mcss.css for the full library, or skip it and build your own components; the layer order declared in mcss.css decides priority either way, so import order between the two doesn’t matter.

Your site’s entry (this site’s is _global.css) pulls in the framework, activates at most one theme, then imports your own CSS unlayered:

/* your entry file */
@import url(./framework/mcss.css);

/* Optional: the mCSS component library. Skip it if you build
   your own components. */
@import url(./framework/mcss.components.css);

/* Optional: activate ONE theme, a swappable style. Theme files are
   self-layered (@layer theme inside the file), so no layer()
   annotation. None imported = the default look. */
/* @import url(./framework/theme.wireframe.css); */

/* Your own CSS: UNLAYERED, after the framework. Its normal
   declarations win over every mCSS layer without fighting
   specificity. One exception: helpers are !important and beat
   unlayered CSS too. */
@import url(./site/global.layout.css);
@import url(./site/component.header.css);
@import url(./site/page.blog.css);
/* etc. */

Settings

Settings are where all custom properties are set.

• Tokens is where you set default values for sizes, font stacks, colors, transitions, etc. See the docs for all the values available by default.
• Interface tokens are an abstraction level to standardize common values across elements and components. For instance, components use --ui-border-color instead of the lower level token --base-200, so one override restyles every bordered surface.
• Media queries include responsive sizes, as well as user preferences like color schemes, reduced motion, etc. See media queries docs.
• Mixins is optional. It is not used in other parts of mCSS by default but can be useful to streamline your own components’ code. It requires a PostCSS plugin to work.

Base

• Simple reset/normalize.

HTML Elements

The default styling of all HTML elements, without classes.

• Sectioning: header, footer, etc.
• Text: a, p, etc.
• Quotes: adds the correct quotes depending on language.
• Media: img,video, etc.
• Table: table, th, etc.
• Form: input, button, etc.
• Interactive: dialog, details.

Global

Global styles included out of the box:

• A responsive grid system.
• A full feature wrapper.
• Common global site layouts.
• Typography, via the .prose class, for long form text, like articles, etc.
• Accessibility (A11Y) specific classes.
• Basic @keyframes animations (e.g., fade in/out)

Components

Self-contained styles for single components, one block per file. The mCSS framework is designed to be used on its own, allowing you to create your own components, but a collection of components built on top of mCSS is included. It ships in its own entry file (mcss.components.css, or dist/mcss.components.css as a drop-in): mcss.css only declares the components layer slot, so the library is opt-in.

Some components are CSS-only: a single class on a single element, with no Astro component because there is no markup structure to encapsulate. For example, the .badge class on a <span> is the whole badge, and the .bt class can style a <button> or an <a> (styling that has to be opt-in, since not all links are buttons). Their docs live in the components section like everything else, marked “CSS-only”.

Theme

This is the one layer the framework declares but never fills: mcss.css imports no theme, and the default look IS the empty theme layer. The theme itself is really part of your CSS: one theme.*.css file of token overrides plus optional style rules that you activate from your own entry file, whether it’s a shipped theme like theme.wireframe.css or one you wrote yourself.

The theme’s remit is wider than component classes. Theme files are the ONE place allowed to select framework classes from outside: component classes, framework globals like .a11y-skipLink, and bare element selectors. A body rule belongs in the theme, not in your project CSS. Before adding a declaration to body, check whether an interface token already delivers it: the elements layer paints the page from --body-background-color, --text-color, and --text, so a theme that sets those does not also need to set background-color, color, and font-family. See the theme docs for how themes work and how to build one.

Helpers

Helpers provide classes for “one-off” local overrides. They’re similar to utility classes from other frameworks, with a critical difference: they’re meant to be used as a last resort and as sparingly as possible. Helper declarations use !important, and important declarations inside a layer beat unlayered CSS, so a helper class overrides everything on that element, including your own CSS. Read the docs for more info on this.

Your CSS (unlayered)

Your own CSS (components, page styles, third-party CSS you don’t control) is imported after the mCSS framework. Your CSS is unlayered. It wins over all layered mCSS files. You never have to worry about the framework’s specificity. The one exception is helper classes: their !important declarations win over everything, on purpose.

Your CSS goes inside your site/ folder, in this order:

• theme.*.css is the one file of yours that is not unlayered: a theme wraps its own content in @layer theme, deliberately dropping into the framework’s theme slot so it restyles the framework defaults while the rest of your CSS below still beats it.
• global.*.css for site-wide additions to the framework’s globals.
• component.*.css for your own blocks (most of your CSS should end up here)
• page.*.css for the rare page-specific styles
• external.*.css for CSS you don’t control, i.e. plugins

Because those files are unlayered, the order you import them in is important. Make sure you have a clear mental model of ITCSS principles before delving in. You can also refer to mCSS own /site’s setup as a reference.

Pages

Page files belong to your /site folder. They’re used sparingly, for the rare page-specific overrides.

.blog-index {
  --layout-content-width: 100%;
}

Note: page classes should go on the <body> in the HTML, at the same level as the layout scaffold classes (not on <main>):

<body class="layout layout-centered blog blog-index"></body>

Print styles

Two rules bend for one file: one block per file, and the rule that a selector only contains classes from its own block. global.print.css holds every @media print rule in the project, including rules that select classes belonging to other blocks.

Print is not a variation on the screen design, it is a different artifact. It removes the parts of the interface that paper cannot use, flattens tinted surfaces that cost toner and print as nothing, and decides where the page may break. That is a single decision about a medium, and a decision is easier to keep coherent in one place than spread across every component it touches. Reading global.print.css top to bottom should tell you what a printed page looks like; you cannot read that out of fourteen @media print blocks in fourteen files.

Group it by intent rather than by component: the sheet, what paper cannot use, ink instead of tint, where the page may break. Grouping that way also collapses repeated rules into shared selector lists, which a per-component split cannot do.

@media print {
  /* What paper cannot use */
  .a11y-skipLink,
  .header,
  .footer,
  .signup {
    display: none;
  }

  /* Ink instead of tint */
  .codeBlock_pre,
  .caveat,
  .correction {
    background: none;
    /* --print-rule is yours to define, in your theme */
    border: 1pt solid var(--print-rule);
  }
}

The cost of the exception is that deleting a component can leave an orphan rule behind. Naming the exception is what makes that affordable: there is one file to scan, not fourteen.

No other media query gets this exception. @media (forced-colors: active), prefers-reduced-motion, and prefers-color-scheme stay in the file of the block they concern. Those are per-block statements about how one component survives a condition: a note on that component, and part of understanding it. A print rule is one line in the design of a printed page, and belongs with the rest of that design.

Classes in mCSS are inspired by BEM, but they’re simpler to use and easier to look at.

You can use standard BEM, the even more verbose BEMIT, or any other syntax you’d like. But the mCSS syntax gets you 90% of the same benefits without any drawbacks, even on large projects involving many devs.

Whatever syntax you pick, one rule stands above all of it: never couple classes across components. A selector may only contain classes from its own block, and every override goes through classes you mix onto the markup, component tokens, or the theme. It’s important enough to have its own article, with plenty of examples.

Blocks, elements, and modifiers

States

Here’s how you implement a state in BEM vs. mCSS.

HTML:

<!-- BEM -->
<form class="form">
  <input class="form__input" type="text" />
  <input class="form__submit form__submit--disabled" type="submit" />
</form>

<!-- mCSS -->
<form class="form">
  <input class="form_input" type="text" />
  <input class="form_submit is-disabled" type="submit" />
</form>

CSS:

/* BEM */
.form__submit--disabled {
}

/* mCSS */
.form_submit {
  &.is-disabled {
  }
}

One gotcha to keep in mind: nesting a state inside its block compiles to a two-class selector (.form_submit.is-disabled), which beats single-class modifiers like .form_submit-lg. That’s usually what you want: states describe what’s happening right now, so they should win. But it means a state block is the wrong place to declare default values for custom properties. Declare defaults on the block itself, let modifiers override them, and only use the properties inside the state. Otherwise your size and color modifiers will mysteriously stop working the moment the state class shows up.

.avatar {
  /* ✅ defaults live on the block… */
  --status-dot-size: 12px;

  &.is-online {
    /* ❌ …not here: this would override .avatar-xl's dot size */
    &::after {
      /* ✅ states just consume the properties */
      width: var(--status-dot-size);
    }
  }
}

.avatar-xl {
  /* modifiers can now override the default */
  --status-dot-size: 20px;
}

Targeting HTML elements within a component

When using mCSS syntax within a component, it’s considered overkill to create a class for every single HTML element you need to target.

For example, this is acceptable (and recommended) CSS for the tags component (component.tags.css):

.tags {
  li {
    […]
  }

  a {
    […]
    &:hover {
      […]
    }
  }
}

Modifiers and component variations

When using modifiers to create different variations of a component, it’s recommended to use local custom properties if possible instead of overriding the CSS directly.

/* Don't do that */
.component {
  color: blue;
}

.component-variation {
  color: red;
}

/* Do that instead */
.component {
  --color: blue;
  color: var(--color);
}

.component-variation {
  --color: red;
}

See the implementation of the notice component for a real life example.

One block per file

This rule governs every file in your site/ folder, not just component.*.css. Name each file for the block it holds, and hold one block in it. component.card.css is .card, its elements, its modifiers, and its media queries, nothing else. A file carrying a second block is a file you cannot delete when that block dies; a block with no file of its own is a block nobody can find.

• A container’s items are elements, not blocks: a step indicator only ever exists inside .steps, so it is .steps_step, and it lives in component.steps.css with the rest of its block. The test is whether the thing has a life of its own: a .card that shows up all over the site is its own block, and a .cardGrid built to lay cards out is another, each with its own file.
• A nested component starts its own name, so it gets its own file.
• global.* files are bound by the same rule: one scaffold, one utility, one role family per file. A global.layout.css holding the page shell, a section pattern, and nine typography classes is three files.

The import list in your entry file is then also the list of blocks in your project, in build order, which makes an unfamiliar codebase readable from a single file.

Any questions or feedback? Head over to the Github discussions!

mCSS targets Baseline 2024: it freely uses cascade layers (@layer), nesting, :has(), light-dark(), @scope, text-wrap: balance, and relative color syntax, all natively, with no polyfills. In practice that means Chrome and Edge 130, Firefox 132, and Safari 18.2 or newer, the versions that shipped by late 2024.

How things degrade in older browsers:

• Cascade layers, nesting, :has(), light-dark(): load-bearing. Browsers without them are not supported. light-dark() deserves a specific warning: every color token stores it in a custom property, and a custom property holding an unsupported function does not fall back, it makes the properties that consume it invalid at computed-value time. Browsers older than mid-2024 get unset colors (currentColor borders, transparent backgrounds), not the light palette.
• @scope (mid-2024): only affects .prose internals; spacing between prose blocks and headings loses one refinement.
• Scroll-driven animations: the ReadProgressBar component detects support and falls back to JavaScript automatically.
• sibling-index(), CSS random(), SVG displacement filters: used only by the wireframe theme for per-element sketch variation; without them the theme keeps its fixed hand-drawn look.

The exact compile floor lives in .browserslistrc (baseline 2024). Keep it: with a broader target, postcss-preset-env starts polyfilling the features above, which costs real weight (a light-dark() polyfill alone added 23% to a measured build) and quietly rewrites CSS the framework’s layer model depends on.