styleguide/web/web.md - chromium/src - Git at Google

 <style>

 .note::before {
   content: 'Note: ';
   font-variant: small-caps;
   font-style: italic;
 }

 </style>

 # 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](../c++/c++.md).

 This guide follows and builds on:

 * [Google HTML/CSS Style Guide](https://google.github.io/styleguide/htmlcssguide.html)
 * [Google JavaScript Style Guide](https://google.github.io/styleguide/jsguide.html)
 * [Google Polymer Style Guide](http://go/polymer-style)

 <div class="note">
 Concerns for browser compatibility are usually not relevant for Chromium-only
 code.
 </div>

 ## 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:
 ```html
 <!-- missile-button.html -->
 <link rel="stylesheet" href="warnings.css">
 <b class="warning">LAUNCH BUTTON WARNING</b>
 <script src="missile-button.js">
 ```
 ```css
 /* warnings.css */
 .warning {
   color: red;
 }
 ```
 ```js
 // missile-button.js
 document.querySelector('b').onclick = fireZeeMissiles;
 ```

 DON'T:
 ```html
 <!-- missile-button.html -->
 <b style="color: red;" onclick="fireZeeMissiles()">LAUNCH BUTTON WARNING</span>
 ```

 <div class="note">
 For various technical and historical reasons, code using the Polymer library may
 use <code>on-event</code>-style event listener wiring and
 <code>&lt;style&gt;</code> tags that live inside of .html files.
 </div>

 ## HTML

 See the [Google HTML/CSS Style
 guide](https://google.github.io/styleguide/htmlcssguide.html).

 ### Head

 ```html
 <!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.

 <div class="note">
 Polymer event handlers like <code>on-tap</code> are allowed and often reduce the
 amount of addressing (adding an ID just to wire up event handling).
 </div>

 ### Body

 ```html
 <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" />`

 <div class="note">
 All <code>&lt;custom-elements&gt;</code> and some HTML elements like
 <code>&lt;iframe&gt;</code> require closing.
 </div>

 * 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](https://google.github.io/styleguide/htmlcssguide.html) (and again, browser
 compatibility issues are less relevant for Chrome-only code).

 ```css
 .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` and `--mixins: {}` 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:
 ```css
 @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


 ### 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`) because alpha can be more easily added.
     * 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 grit's flattening.

 ```css
 background-image: url(../path/to/image.svg);
 ```

 The contents of file.png are base64-encoded and the `url()` is replaced with

 ```css
 background-image: url(data:image/svg+xml;base64,...);
 ```

 if `flattenhtml="true"` is specified in your .grd file.

 ### RTL

 ```css
 .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](https://google.github.io/styleguide/jsguide.html).

 * Use `$('element-id')` instead of `document.getElementById`

 * Use single-quotes instead of double-quotes for all strings.
     * `clang-format` now handles this automatically.

 * Omit curly braces for single-line if statements.

 * 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](https://developers.google.com/closure/compiler/docs/js-for-compiler)
   for @ directives

 * Prefer `event.preventDefault()` to `return false` from event handlers

 ### Closure compiler

 * Use the [closure
   compiler](https://chromium.googlesource.com/chromium/src/+/master/docs/closure_compilation.md)
   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>`

 ### Events

 * Use Polymer's `on-tap` for click events instead of `on-click`
     * `on-tap` handlers should use `stopPropagation()` to prevent parents from
       handling the event where appropriate.

 <div class="note">
 Calling <code>stopPropagation()</code> from an <code>on-tap</code> handler will
 not prevent on-click event handlers, so make sure that <i>on-tap</i> is used
 consistently throughout the page.
 </div>

 ## Polymer

 Also see the [Google Polymer Style Guide](http://go/polymer-style).

 * Use a consistent ordering in the “prototype” object passed to `Polymer()`:
     * `is`
     * `behaviors`
     * `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`).

 ```js
 properties: {
   foo: {type: Number, observer: 'fooChanged_'}
 },

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

 * Make good use of the  [`dom-if` template](
 https://www.polymer-project.org/2.0/docs/devguide/templates#dom-if):
   * Consider using `dom-if` to lazily render parts of the DOM that are hidden by
   default. Also consider using [`cr-lazy-render`](
   https://cs.chromium.org/chromium/src/ui/webui/resources/cr_elements/cr_lazy_render/cr_lazy_render.js)
   instead.
   * **Only use`dom-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](
     https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden)
     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](https://github.com/PolymerElements/iron-icons/blob/master/iron-icons.html).

 ## Grit processing

 Grit is a tool that runs at compile time to pack resources together into
 Chromium.

 ### Preprocessing

 Grit can be used to selectively include or exclude code at compile-time in web
 code.  Preprocessing is be enabled by adding the `preprocess="true"` attribute
 inside of a `.grd` file on `<structure>` and `<include>` nodes.

 <div class="note">
 These preprocesor statements can live in places that surprise linters or
 formatters (for example: running clang-format on a .js file with an &lt;if&gt;
 in it).  Generally, putting these language-invalid features inside of comments
 helps alleviate problems with unexpected input.
 </div>

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

 Example:
 ```js
 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>` to close these tags; they're not needed nor supported.

 <div class="note">
 Using <code>&lt;include&gt;</code> simply pastes the entire contents of a file,
 which can lead to duplication.  If you simply want to ensure some code is loaded
 (and usually you do), you should use HTML Imports instead.
 </div>

 Grit can read and inline resources when enabled via `flattenhtml="true"`.

 <div class="note">
 The implementation of flattening does HTML parsing and URL detection via regular
 expresions and is not guaranteed to work in all cases.
 </div>

 Example:

 ```css
 .spinner {
   background: url(../relative/file/path/to/spinner.svg);
 }
 ```

 Is transformed to:

 ```css
 .spinner {
   background: url(data:image/svg+xml;... base64-encoded content ...);
 }
 ```

 A minification tool can be specified to Grit (like Closure compiler) to
 transform the code before it's packed into a bundle.
	<style>

	.note::before {
	content: 'Note: ';
	font-variant: small-caps;
	font-style: italic;
	}

	</style>

	# 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](../c++/c++.md).

	This guide follows and builds on:

	* [Google HTML/CSS Style Guide](https://google.github.io/styleguide/htmlcssguide.html)
	* [Google JavaScript Style Guide](https://google.github.io/styleguide/jsguide.html)
	* [Google Polymer Style Guide](http://go/polymer-style)

	<div class="note">
	Concerns for browser compatibility are usually not relevant for Chromium-only
	code.
	</div>

	## 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:
	```html
	<!-- missile-button.html -->
	<link rel="stylesheet" href="warnings.css">
	<b class="warning">LAUNCH BUTTON WARNING</b>
	<script src="missile-button.js">
	```
	```css
	/* warnings.css */
	.warning {
	color: red;
	}
	```
	```js
	// missile-button.js
	document.querySelector('b').onclick = fireZeeMissiles;
	```

	DON'T:
	```html
	<!-- missile-button.html -->
	<b style="color: red;" onclick="fireZeeMissiles()">LAUNCH BUTTON WARNING</span>
	```

	<div class="note">
	For various technical and historical reasons, code using the Polymer library may
	use <code>on-event</code>-style event listener wiring and
	<code><style></code> tags that live inside of .html files.
	</div>

	## HTML

	See the [Google HTML/CSS Style
	guide](https://google.github.io/styleguide/htmlcssguide.html).

	### Head

	```html
	<!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.

	<div class="note">
	Polymer event handlers like <code>on-tap</code> are allowed and often reduce the
	amount of addressing (adding an ID just to wire up event handling).
	</div>

	### Body

	```html
	<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" />`

	<div class="note">
	All <code><custom-elements></code> and some HTML elements like
	<code><iframe></code> require closing.
	</div>

	* 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](https://google.github.io/styleguide/htmlcssguide.html) (and again, browser
	compatibility issues are less relevant for Chrome-only code).

	```css
	.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` and `--mixins: {}` 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:
	```css
	@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


	### 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`) because alpha can be more easily added.
	* 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 grit's flattening.

	```css
	background-image: url(../path/to/image.svg);
	```

	The contents of file.png are base64-encoded and the `url()` is replaced with

	```css
	background-image: url(data:image/svg+xml;base64,...);
	```

	if `flattenhtml="true"` is specified in your .grd file.

	### RTL

	```css
	.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](https://google.github.io/styleguide/jsguide.html).

	* Use `$('element-id')` instead of `document.getElementById`

	* Use single-quotes instead of double-quotes for all strings.
	* `clang-format` now handles this automatically.

	* Omit curly braces for single-line if statements.

	* 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](https://developers.google.com/closure/compiler/docs/js-for-compiler)
	for @ directives

	* Prefer `event.preventDefault()` to `return false` from event handlers

	### Closure compiler

	* Use the [closure
	compiler](https://chromium.googlesource.com/chromium/src/+/master/docs/closure_compilation.md)
	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>`

	### Events

	* Use Polymer's `on-tap` for click events instead of `on-click`
	* `on-tap` handlers should use `stopPropagation()` to prevent parents from
	handling the event where appropriate.

	<div class="note">
	Calling <code>stopPropagation()</code> from an <code>on-tap</code> handler will
	not prevent on-click event handlers, so make sure that <i>on-tap</i> is used
	consistently throughout the page.
	</div>

	## Polymer

	Also see the [Google Polymer Style Guide](http://go/polymer-style).

	* Use a consistent ordering in the “prototype” object passed to `Polymer()`:
	* `is`
	* `behaviors`
	* `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`).

	```js
	properties: {
	foo: {type: Number, observer: 'fooChanged_'}
	},

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

	* Make good use of the [`dom-if` template](
	https://www.polymer-project.org/2.0/docs/devguide/templates#dom-if):
	* Consider using `dom-if` to lazily render parts of the DOM that are hidden by
	default. Also consider using [`cr-lazy-render`](
	https://cs.chromium.org/chromium/src/ui/webui/resources/cr_elements/cr_lazy_render/cr_lazy_render.js)
	instead.
	* Only use`dom-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](
	https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden)
	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](https://github.com/PolymerElements/iron-icons/blob/master/iron-icons.html).

	## Grit processing

	Grit is a tool that runs at compile time to pack resources together into
	Chromium.

	### Preprocessing

	Grit can be used to selectively include or exclude code at compile-time in web
	code. Preprocessing is be enabled by adding the `preprocess="true"` attribute
	inside of a `.grd` file on `<structure>` and `<include>` nodes.

	<div class="note">
	These preprocesor 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.
	</div>

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

	Example:
	```js
	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>` to close these tags; they're not needed nor supported.

	<div class="note">
	Using <code><include></code> simply pastes the entire contents of a file,
	which can lead to duplication. If you simply want to ensure some code is loaded
	(and usually you do), you should use HTML Imports instead.
	</div>

	Grit can read and inline resources when enabled via `flattenhtml="true"`.

	<div class="note">
	The implementation of flattening does HTML parsing and URL detection via regular
	expresions and is not guaranteed to work in all cases.
	</div>

	Example:

	```css
	.spinner {
	background: url(../relative/file/path/to/spinner.svg);
	}
	```

	Is transformed to:

	```css
	.spinner {
	background: url(data:image/svg+xml;... base64-encoded content ...);
	}
	```

	A minification tool can be specified to Grit (like Closure compiler) to
	transform the code before it's packed into a bundle.