Chromium Web Development Style Guide

[TOC]

Where does this style guide apply?

This style guide targets Chromium frontend features implemented with JavaScript, CSS, and HTML. Developers of these features should adhere to the following rules where possible, just like those using C++ conform to the Chromium C++ styleguide.

This guide follows and builds on:

*** aside Note: Concerns for browser compatibility are usually not relevant for Chromium-only code.

Separation of presentation and content

When designing a feature with web technologies, separate the:

content you are presenting to the user (HTML)
styling of the data (CSS)
logic that controls the dynamic behavior of the content and presentation (JS)

This highlights the concern of each part of the code and promotes looser coupling (which makes refactor easier down the road).

Another way to envision this principle is using the MVC pattern:

MVC Component	Web Component
Model	HTML
View	CSS
Controller	JS

It's also often appropriate to separate each implementation into separate files.

DO:

<!-- missile-button.html -->
<link rel="stylesheet" href="warnings.css">
<b class="warning">LAUNCH BUTTON WARNING</b>
<script src="missile-button.js">

/* warnings.css */
.warning {
  color: red;
}

// missile-button.js
document.querySelector('b').onclick = fireZeeMissiles;

DON'T:

<!-- missile-button.html -->
<b style="color: red;" onclick="fireZeeMissiles()">LAUNCH BUTTON WARNING</b>

*** aside Note: For various technical and historical reasons, code using the Polymer library may use on-event-style event listener wiring and <style> tags that live inside of .html files.

HTML

See the Google HTML/CSS Style guide.

Head

<!doctype html>
<html dir="$i18n{direction}">
<head>
  <meta charset="utf-8">
  <title>$i18n{myFeatureTitle}</title>
  <link rel="icon" href="feature.png">
  <link rel="stylesheet" href="feature.css">
  <script src="feature.js"></script>
</head>
…
</html>

Specify <!doctype html>.
Set the dir attribute of the html element to the localized ‘textdirection’ value. This flips the page visually for RTL languages and allows html[dir=rtl] selectors to work.
Specify the charset, UTF-8.
Link in image, icon and stylesheet resources.
- Do not style elements with style="..." attributes.
Include the appropriate JS scripts.
- Do not add JS to element event handlers.

*** aside Note: Polymer event handlers like on-click are allowed and often reduce the amount of addressing (adding an ID just to wire up event handling).

Body

<h3>$i18n{autofillAddresses}</h3>
<div class="settings-list">
  <list id="address-list"></list>
  <div>
    <button id="autofill-add-address">$i18n{autofillAddAddress}</button>
  </div>
</div>
<if expr="chromeos">
  <a href="https://www.google.com/support/chromeos/bin/answer.py?answer=142893"
      target="_blank">$i18n{learnMore}</a>
</if>

Element IDs use dash-form
- Exception: camelCase is allowed in Polymer code for easier this.$.idName access.
Localize all strings using $i18n{}
Use camelCase for $i18n{} keys names.
Add 2 space indentation in each new block.
Adhere to the 80-column limit.
- Indent 4 spaces when wrapping a previous line.
Use double-quotes instead of single-quotes for all attributes.
Don't close single tags
- DO: <input type="radio">
- DON'T: <input type="radio" />

*** aside Note: All <custom-elements> and some HTML elements like <iframe> require closing.

Use the button element instead of <input type="button">.
Do not use <br>; place blocking elements (<div>) as appropriate.
Do not use spacing-only divs; set the margins on the surrounding elements.
Only use <table> elements when displaying tabular data.
Do not use the for attribute of <label>
- If you're labelling a checkbox, put the <input> inside the <label>
- If you're labelling purely for accessibility, e.g. a <select>, use aria-labelledby

CSS

See the Google HTML/CSS style guide (and again, browser compatibility issues are less relevant for Chrome-only code).

.raw-button,
.raw-button:hover,
.raw-button:active {
  --sky-color: blue;
  -webkit-margin-collapse: discard;
  background-color: rgb(253, 123, 42);
  background-repeat: no-repeat;
  border: none;
  min-width: 0;
  padding: 1px 6px;
}

Specify one selector per line.
- Exception: One rule / one line frames in a @keyframe (see below).
Opening brace on the same line as the last (or only) selector.
Two-space indentation for each declaration, one declaration per line, terminated by a semicolon.
Use shorthand notation when possible.
Alphabetize properties.
- -webkit properties should be listed at the top, sorted alphabetically.
- --variables should be alphabetically declared when possible.
Insert a space after the colon separating property and value.
Do not create a class for only one element; use the element ID instead.
When specifying length values, do not specify units for a zero value, e.g., left: 0px; becomes left: 0;
- Exception: 0% values in lists of percentages like hsl(5, 0%, 90%) or within @keyframe directives, e.g:

@keyframe animation-name {
  0% { /* beginning of animation */ }
  100% { /* end of animation */ }
}

Use single quotes instead of double quotes for all strings.
Don't use quotes around url()s unless needed (i.e. a data: URI).
Class names use dash-form.
If time lengths are less than 1 second, use millisecond granularity.
- DO: transition: height 200ms;
- DON'T: transition: height 0.2s;
Use two colons when addressing a pseudo-element (i.e. ::after, ::before, ::-webkit-scrollbar).
Use scalable font-size units like % or em to respect users' default font size
Don't use CSS Mixins (--mixin: {} or @apply --mixin;) in new code. We're removing them.
- Mixins were dropped from CSS in favor of CSS Shadow Parts.
- Instead, replace CSS mixin usage with one of these natively supported alternatives:
  - CSS Shadow Parts or CSS variables for styling of DOM nodes residing in the Shadow DOM of a child node.
  - Plain CSS classes, for grouping a set of styles together for easy reuse.

Color

When possible, use named colors (i.e. white, black) to enhance readability.
Prefer rgb() or rgba() with decimal values instead of hex notation (#rrggbb).
- Exception: shades of gray (i.e. #333)
If the hex value is #rrggbb, use the shorthand notation #rgb.

URLs

Don't embed data URIs in source files. Instead, use a relative path to an icon in your UI (and include this icon in the generated grd file), or use an absolute URL for an icon from the shared resources at ui/webui/resources:

background-image: url(chrome://resources/images/path/to/image.svg);

RTL

.suboption {
  margin-inline-start: 16px;
}

#save-button {
  color: #fff;
  left: 10px;
}

html[dir='rtl'] #save-button {
  right: 10px;
}

Use RTL-friendly versions of things like margin or padding where possible:

margin-left -> margin-inline-start
padding-right -> padding-inline-end
text-align: left -> text-align: start
text-align: right -> text-align: end
set both left for [dir='ltr'] and right for [dir='rtl']

For properties that don't have an RTL-friendly alternatives, use html[dir='rtl'] as a prefix in your selectors.

JavaScript

Style

See the Google JavaScript Style Guide as well as ECMAScript Features in Chromium.

Use $('element-id') instead of document.getElementById. This function can be imported from util.m.js.
Use single-quotes instead of double-quotes for all strings.
- clang-format now handles this automatically.
Use ES5 getters and setters
- Use @type (instead of @return or @param) for JSDoc annotations on getters/setters
See Annotating JavaScript for the Closure Compiler for @ directives
Prefer event.preventDefault() to return false from event handlers

Closure compiler

Use the closure compiler to identify JS type errors and enforce correct JSDoc annotations.
Add a BUILD.gn file to any new web UI code directory.
Ensure that your BUILD.gn file is included in src/BUILD.gn:webui_closure_compile (or somewhere in its deps hierarchy) so that your code is typechecked in an automated way.
Type Polymer elements by appending 'Element' to the element name, e.g. /** @type {IronIconElement} */
Use explicit nullability in JSDoc type information
- Rather than @type {Object} use:
  - {!Object} for only Object
  - {!Object|undefined} for an Object that may be undefined
  - {?Object} for Object that may be null
- Do the same for typedefs and Array (or any other nullable type)
Don't add a . after template types
- DO: Array<number>
- DON'T: Array.<number>
Don't specify string in template object types. That's the only type of key Object can possibly have.
- DO: Object<T>
- DON'T: Object<string, T>
Use template types for any class that supports them, for example:
- Array
- CustomEvent
- Map
- Promise
- Set

Polymer

Also see the Google Polymer Style Guide.

Elements with UI should have their HTML in a .html file and logic in a JS file with the same name. The HTML should be copied into the final JS file at build time, replacing the special {__html_template__} sequence, using the html_to_js BUILD.gn rule. For example the following will paste the contents of my_app.html into the final generated JS file:

  html_to_js('web_components') {
    js_files = [ 'my_app.js' ]
  }

In new code, use class based syntax for custom elements. Example:

import {html, PolymerElement} from 'chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js';

class MyAppElement extends PolymerElement {
  static get is() {
    return 'my-app';
  }

  static get template() {
    return html`{__html_template__}`;
  }

  static get properties() {
    return {
      foo: String,
    };
  }
}

customElements.define(MyAppElement.is, MyAppElement);

Use a consistent ordering for common methods (or, in legacy code, the parameters passed to Polymer()):
- is
- behaviors (legacy code only)
- properties (public, then private)
- hostAttributes
- listeners, observers
- created, ready, attached, detached
- public methods
- event handlers, computed functions, and private methods
Use camelCase for element IDs to simplify local DOM accessors (i.e. this.$.camelCase instead of this.$['dash-case']).
Use this.foo instead of newFoo arguments in observers when possible. This makes changing the type of this.foo easier (as the @type is duplicated in less places, i.e. @param).

static get properties() {
  return {
    foo: {type: Number, observer: 'fooChanged_'},
  };
}

/** @private */
fooChanged_() {
  this.bar = this.derive(this.foo);
}

Use native on-click for click events instead of on-tap. 'tap' is a synthetic event provided by Polymer for backward compatibility with some browsers and is not needed by Chrome.
Make good use of the dom-if template:
- Consider using dom-if to lazily render parts of the DOM that are hidden by default. Also consider using cr-lazy-render instead.
- Only usedom-if if the DOM subtree is non-trivial, defined as:
  - Contains more than 10 native elements, OR
  - Contain any custom elements, OR
  - Has many data bindings, OR
  - Includes non-text content (e.g images).
  For trivial DOM subtrees using the HTML hidden attribute yields better performance, than adding a custom dom-if element.
Do not add iron-icons dependency to third_party/polymer/.
- Polymer provides icons via the iron-icons library, but importing each of the iconsets means importing hundreds of SVGs, which is unnecessary because Chrome uses only a small subset.
- Alternatives:
  - Include the SVG in a WebUI page-specific icon file. e.g. chrome/browser/resources/settings/icons.html.
  - If reused across multiple WebUI pages, include the SVG in ui/webui/resources/cr_elements/icons.html .
- You may copy the SVG code from iron-icons files.

Grit processing

Grit is a tool that runs at compile time to pack resources together into Chromium. Resources are packed from grd files. Most Chromium WebUI resources should be located in autogenerated grd files created by the generate_grd gn rule.

Preprocessing

Sometimes it is helpful to selectively include or exclude code at compile-time. This is done using the preprocess_if_expr gn rule, which makes use of a subset of grit that reads and processes files for <if expr> without running the entire grit resource packing process. Files that require preprocessing are passed to the rule as in_files. Preprocessed versions with the same names will be written to the specified out_folder and are listed in out_manifest, which can be passed to the generate_grd rule to generate entries for them in a grd file.

Example

The following BUILD.gn example code uses preprocess_if_expr to preprocess any <if expr> in the final my_app.js file that is generated by the earlier html_to_js example. It then uses the manifest from this operation and the in_files option to place both the final, preprocessed file and a separate (not preprocessed) icon into a generated grd file using generate_grd:

preprocess_folder = "preprocessed"
preprocess_manifest = "preprocessed_manifest.json"

# Read file from target_gen_dir, where it will be pasted by html_to_js.
preprocess_if_expr("preprocess") {
  deps = [ ":web_components" ]
  in_folder = target_gen_dir
  in_files = [ "my_app.js" ]
  out_folder = "$target_gen_dir/$preprocess_folder"
  out_manifest = "$target_gen_dir/$preprocess_manifest"
}

# Put the preprocessed file as well as a separate my_icon.svg file in the grd:
generate_grd("build_grd") {
  input_files = [ "my_icon.svg" ]
  input_files_base_dir = rebase_path(".", "//")
  deps = [ ":preprocess" ]
  manifest_files = [ "$target_gen_dir/$preprocess_manifest" ]
  grd_prefix = [ "foo" ]
  out_grd = "$target_gen_dir/resources.grd"
}

*** aside Note #1: In a few legacy resources, preprocessing is enabled by adding the preprocess="true" attribute inside of a .grd file on <structure> and <include> nodes.

Note #2: These preprocessor statements can live in places that surprise linters or formatters (for example: running clang-format on a .js file with an <if> in it). Generally, putting these language-invalid features inside of comments helps alleviate problems with unexpected input.

<if> tags allow conditional logic by evaluating an expression in a compile-time environment of grit variables. These allow conditionally including or excluding code.

Example:

function isWindows() {
  // <if expr="win">
  return true;
  // </if>
  return false;
}

<include src="[path]"> reads the file at path and replaces the <include> tag with the file contents of [path]. Don't use <include> in new JS code; it is being removed. Instead, use JS imports. If there is concern about importing a large number of JS files, the optimize_webui build rule supports bundling pages using Rollup.

Some legacy UIs use Grit to read and inline resources via flattenhtml="true". This option should not be used in new code; instead, use JS imports and bundling as needed. Icons can also be placed in an iconset, to avoid importing them individually.

*** aside Note: The implementation of flattening does HTML parsing and URL detection via regular expressions and is not guaranteed to work in all cases. In particular, it does not work with any generated resources.

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Chromium Web Development Style Guide

Where does this style guide apply?

Separation of presentation and content

HTML

Head

Body

CSS

Color

URLs

RTL

JavaScript

Style

Closure compiler

Polymer

Grit processing

Preprocessing

Example

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Chromium Web Development Style Guide

Where does this style guide apply?

Separation of presentation and content

HTML

Head

Body

CSS

Color

URLs

RTL

JavaScript

Style

Closure compiler

Polymer

Grit processing

Preprocessing

Example