ZASM: Zero-Ambiguity Stylesheet Methodology


In this document I hope to define a CSS methodology using a small set of unambigous rules. This methodology has these aims:

  • extreme maintainability
    • high readability
    • guided writability
    • low barrier to adding collaborators
  • broad compatibility
    • with mobile-first patterns
    • with responsive designs
    • with pre-processors and build tools
    • with modern web standards
    • with backwards compatibility
    • with legacy codebases as well as green-field projects

The first major goal is maintainability — important whether your project involves one person or ten. For any sort of styling you want to do, under ZASM, there should be one clear, “right’ way to write the selector for it; this makes the CSS easy to author. Class names written under ZASM are designed to be instantly identifiable as belonging to one of the categories defined in the methodology. This provides readability and helps communicate author intention. By keeping the methodology small and tightly defined, new collaborators won’t face a large uphill climb to productivity — at least, assuming they have already learned enough CSS, an admittedly complicated pre-requisite!

The second top-level goal is broad compatibility. That means that ZASM should be able to be the one methodology you can reach for in all your projects, both shared and personal. To do this, we carefully establish rules that are strict enough to provide some built in ‘defenses’ as well as flexible enough to adapt to any specific project.

Base Styles

In as little CSS as possible, and by writing selectors without any class names at all (pseudo-classes are OK), set base element styles including your preferred font stack.

It is recommended to set a good base line height. It is up to you whether that is a ratio (1.2, 1.6, etc) or an absolute measurement (24px).

If desired, style links and form controls (input elements). This is not necessary because you can instead ensure every link and control is styled by a module; it is up to your preference.

In your base styles, fix dumb browser default styling issues. Optionally set sane defaults for box-sizing and overflow-wrap on everything.

If using CSS variables, set globally applicable variables, or even all variables, here. Your color palette is the most obvious thing to define here. Optionally modify these using media queries and/or classes applied to html (aka :root). It is recommended that any such class names start with theme-: theme-dark.

Notably, it is perfectly okay under ZASM if you avoid CSS variables altogether. You do not need to support multiple themes at all. Or you can ship a hundred different themes all compiled as different .css files using Sass. It doesn’t affect the rest of the methodology at all.

ZASM recommends writing fewer base styles and more modules.

:root {
  --page-bg-color: #def;
  --text-color: #222;

/* Automatically use dark mode if the user prefers it */
@media (prefers-color-scheme: dark) {
  :root {
    --page-bg-color: #123;
    --text-color: #eee;

/* Note, redundant variables to automatic dark mode for when
 * dark mode is manually applied to <html> by class */
:root.theme-dark {
  --page-bg-color: #123;
  --text-color: #eee;

:root.theme-halloween {
  --page-bg-color: black;
  --text-color: orangered;

:root, body, input, textarea, button {
  font: 16px/1.4 "Brand Font", system-ui, sans-serif;

* {
  box-sizing: border-box;
  overflow-wrap: break-word;

:link { color: blue; }
:visited { color: rebeccapurple; }
:link:active, :visited:active { color: var(--text-color);


Modules are things — just about anything. They are composable and can be placed within each other (when it makes sense). They can be very complicated. They can have parts and variants.

The naming rule for a module's class name (or "base class") is: any three-letter class name.

However, you should add a prefix such as m- for all module names when introducing ZASM to codebases with existing HTML that uses unprefixed class names: .m-mdl instead of .mdl and .m-mdl__header instead of .mdl__header.

If your organization is adopting ZASM, there may be cases where you may not desire or be able to follow the three-letter naming rule. That is an acceptable deviation; just remember to make your module naming rule clear enough that it is readily understood to be a module when it is seen in use. And try to avoid long module names as it makes naming parts and variants very clunky.

.mdl { /* Base class for a "Modal" module */ }
.mdl__header { /* A part called "header" within an "mdl" modal */ }
.mdl--alert { /* Variant of "mdl" fulfilling "alert" purpose */ }


In ZASM, a "part" is what BEM calls an "element": It's something that belongs within its module. I call them "parts" because "element" already has a different meaning in CSS and HTML.

Parts are always named like this: base module name + two underscores + kebab-cased part name.

Example: Module foo could have parts foo__intro and foo__author-signature.

A module can have unlimited parts. Parts can be within other parts — for example you might have a close button .mdl__close within the modal header .mdl__header. Whether the parts of the module are required or optional is up to you.


"Variants" are variations of a module. They are what BEM calls "modifiers" and what SMACSS calls "subclassed modules."

The class name for a variant is: base module name + two hyphens + kebab-cased variant name.

Example: Module foo could have a variant in-progress, so the class name would be foo--in-progress (assuming you are not prefixing module names with m-).

Multiple variants can be applied to a module simultaneously, if it makes sense. You can style these intersections when needed:

.msg { /* message module */ }
.msg--loading { color: gray; }
.msg--mine { color: blue; }
.msg--loading.msg--mine { color: slategray; }

Variants can, but do not necessarily, represent states. They can also transform the module as desired for placement elsewhere. Or they can simply change the appearance to grab more or less visual attention. The classic example is btn, a normal button, and btn--primary, a 'primary' button with strong contrast to the surrounding area due its purpose as the primary action on the screen.

Do apply multiple variant classes to an element as appropriate:

  class="btn btn--primary btn--disabled"
  value="Submit" />

<section role="alert" class="mdl mdl--alert mdl--open">
  <header class="mdl__header"> ... </header>
  <div class="mdl__body"> ... </div>

Do not add the base module class to selectors unnecessarily: Style .btn--primary, not .btn.btn--primary.

Layout Modules

One valid use of a module is to create one or more containers for laying out content (= other modules).

There is no special naming designation for layout modules. In fact, a module can handle its own layout as well as doing other things. You do not need to decide whether a module is a layout module or a regular module; it can be both. Or you can separate the concerns if that increases your re-usability.

You can use the wildcard direct child selector (> *) in layout modules, but it is even better to swap this for named parts.

.xyz { /* module that lays out its children */ }
.xyz > * { flex: 1; } /* Acceptable selector */
.xyz__child { flex: 1; } /* Even better selector! */

ZASM strongly supports responsive design, which is what we call it when the same page, with the same markup, effortly resizes and rearranges itself to fit screens of any size and shape. ZASM avoids simple "layout" classes that always do the same thing (e.g., apply two columns). Instead we empower modules to lay out their parts however they want. That will often mean using @media queries to switch to a compact, single-column layout on narrow screens. Or it can mean using flexbox-based responsive techniques. Or liberal use of calc(). Or all of the above. It's up to you!

Caution: Never write CSS with descendant and element name seletors like this for a layout module:

.xyz div { float: left; }

That's a great way to mess up just about every module nested within .xyz, and it’s a completely avoidable violation of ZASM rules.

Content-Oriented Modules

Sometimes you can't realistically apply class names to everything. For instance, the contents of an article probably include lots of headings and paragraphs that don't have class names on them. Or maybe you need to display content generated by a third-party system or a certain software library that has its own ideas about markup.

This is not a problem in ZASM. After all, we have taken care to keep our base styles minimal, and most of our styles only trigger off the specific class names we have invented usng ZASM naming rules.

Cases where you need to bulk-style content that may be using no class names or non-ZASM class names call for content-oriented modules.

These are just a special case of module that is focused on styling its children and descendants.

Imagine a "content" module named cnt.

<article class="cnt">
  <h1>Why My Dog Really Did Eat My Homework</h1>
  <p>Toby is not a bad dog. In fact, ... </p>
.cnt h1 { /* ... */ }
.cnt p { /* ... */ }

Notably content-oriented modules can easily be nested within other modules, but other modules cannot be nested within a content-oriented module without risking styles breaking.

For this reason, content-oriented modules should be relatively few, lest you ruin the composability and predictability of your ZASM-based stylesheet.

By the same logic, you should strive to use the direct child selector (>) whenever possible instead of the descendant selector (space), but this will often be impossible for a well-needed content-oriented module. For example, .cnt > p will work on our example HTML, but it will probably break once a piece of content appears that contains a p using a <section> or <div> tag!


Utility classes should be defined sparingly. You may not even need or write a single utility class in a well-organized ZASM project.

Name them with a u- prefix and kebab-case (hyphenate) the rest.

Utility classes are standalone classes that achieve a narrow purpose. If you are tempted to make a "variant" or "part" of a utility, you should convert it to a module instead.

.u-clear {
  clear: both;

.u-small-caps {
  font-variant: small-caps;

.u-tabular-nums {
  font-variant-numeric: tabular-nums;


  • Never use ancestor or parent selectors to style a module differently based on where it appears. Create a variant instead.

  • Do not invent states (like .is-current). Instead, invent a variant.

  • Do not write layout classes; instead, define a module that has a layout purpose.

  • Do not use !important (unless it is necessary due to some horrible legacy code you can't jettison).

  • Do not write element-based selectors (except for the smallest number necessary in base styles). For example, do not style main or article. Instead, invent a module and apply it. It is acceptable to use ancestor selectors to style elements within a content-oriented module: .cnt p { margin 1.2em 0; } is a fine rule for styling paragraphs within your main content area .cnt.

Non-Styling Class Names

When elements will be targeted via JavaScript (e.g. with querySelector()), prefer a js- prefix: class="js-whatever".

For instrumententation of click tracking, use a consistent prefix such as track-: class="track-home-page-call-to-action".

For enabling automated QA testing, always use class names beginning with qa-: class="qa-home-page-call-to-action".

It is recommended to apply as many non-styling class names to the same element as needed, even if that element also uses one or more ZASM class names for styling purposes.

Cheat Sheet

For quick reference, enjoy the ZASM cheat sheet.


This methodology is hugely inspired by SMACSS and BEM.

This document is open source. Issues and pull requests are welcome.


The Zero-Ambiguity Stylesheet Methodology (ZASM) by Alan J. Hogan is licensed under CC BY 4.0

April 28th, 2022
Alan Hogan (@alanhogan_com).  Contact · About