Input Method Editor API

W3C Working Draft

This version:
Latest published version:
Previous versions:
Latest editor's draft:
Takayoshi Kochi, Google,
Kenji Baheux, Google,
Previous editor:
Hironori Bono (坊野 博典), (formerly) Google


This specification defines an “IME API” that provides Web applications with scripted access to an IME (input-method editor) associated with a hosting user agent. This IME API includes:

This API is designed to be used in conjunction with DOM events [DOM-LEVEL-3-EVENTS].

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is a proposal that is being made available for public review in order to solicit feedback, particularly from implementors, with a goal of potential cross-browser implementation and standardization.

This document was published by the Web Applications Working Group as an Editor's Draft. If you wish to make comments regarding this document, please send them to public-webapps@w3.org with [ime-api] at the start of the subject header (subscribe, archives). All comments are welcome.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

This section is non-normative.

Even though existing Web-platform APIs allow developers to implement very complicated Web applications, such as visual chat applications or WYSIWYG presentation editors, developers have difficulties when implementing Web applications that involve input-method editors. To mitigate the difficulties, the DOM Level 3 Events specification[DOM-LEVEL-3-EVENTS] introduces composition events to retrieve composition text while it is being composed in an associated IME.

However, Web applications can still run into difficulties when they want text input on non-editable elements such as the <div> element without contenteditable attribute being set to true, or interact with IMEs; those difficulties include the fact that a Web application cannot do the following:

To solve these IME-related problems, this specification introduces an IME API that allows Web applications to interact with the IME. This specification introduces interfaces for compositions, so Web applications can read detailed composition data. A Composition object provides a reference to an ongoing IME composition, so Web applications can retrieve the composition text and its attributes. In addition, this API also gives Web applications the ability to give a hint as to where to position a composition window.

There are also proposed standards related to IME:

They can work independently of this API.

Please also refer to the separate use cases for Input Method Editor API document for more information about possible use cases.

Consider the following examples.

This example shows the source which gives a hint for an IME where the application wants it to avoid placing UI elements. A simple web search page which gives a user suggestions while the user is doing composition.

Example 1
<!DOCTYPE html>
<style type="text/css">
#search0 {
    max-width: 400px;

#input0 {
    width: 100%;

#suggest0 {
    width: 100%;
    list-style: none;
    margin: 0;
    padding: 0;
    border-style: solid;
    border-width: 1px;
    border-color: #000;
<script language="javascript" type="text/javascript">
function init() {
    var node = document.getElementById('input0');
    // This code only handles the compositionupdate event for brevity of the
    // example, but of course other input field changes should also be handled.
    node.addEventListener('compositionupdate', onCompositionUpdate, false);

// Sends an XHR request to get search suggestions.
// Upon receiving the result, expandSuggest() is called back.
function getSuggests(query) {
    // For brevity, implementation of this function is omitted.

function expandSuggest(candidates) {
    // Callback after getting search suggestions.
    var suggest = document.getElementById('suggest0');
    var i;
    // Clear old suggestions.
    for (i = 0; i < suggest.childNodes.length; i++) {
    // Render new suggeston list.
    for (i = 0; i < candidates.length; i++) {
        suggest.childNodes[i].textContent = candidates[i];
    // Set exclusion area hint for the IME.
    var input = document.getElementById('input0');
    var context = input.inputMethodContext;
    var relative_x = suggest.offsetLeft - input.offsetLeft;
    var relative_y = suggest.offsetTop - input.offsetTop;
    context.setExclusionRectangle(input, relative_x, relative_y,

function onCompositionUpdate(event) {
    var query = document.getElementById('input0').value;
<div id="search0">
  <input type="text" id="input0" placeholder="search here">
  <ul id="suggest0"></ul>

This example shows the source which draws composition text in its own style.


For the usage of compositionupdate event, see also the note in Drawing Composition Text section.

Example 2
<!DOCTYPE html>
.clause {
  margin-right: 2px;
  border-style: solid;
  border-width: 0px 0px 1px 0px;
  border-color: #333333;
.selected {
  border-width: 0px 0px 2px 0px;
  border-color: #000000;
<script language="javascript" type="text/javascript">
// compositionupdate event handler
function onCompositionUpdate(event) {
    var target = event.target;

    // Remove all children.
    while (target.childNodes.length < 0) {

    // Get the IME context.
    var ctx = target.inputMethodContext;

    // Create clause spans.
    var text = ctx.text;
    var segments = ctx.segments;
    for (i = 0; i < segments.length; i++) {
        var span = document.createElement('span');
        var selected = false;
        if (segments[i] == ctx.selectionStart) {
            selected = true;
        var end = text.length;
        if (i < segments.length - 1) {
             end = segments[i + 1];
        span.textContent = text.substring(ctx.segments[i], end);
        if (selected) {
            ctx.setCaretRectangle(document.body, rect.left, rect.top,
                                                 rect.width, rect.height);

    // Prevent the browser from drawing composition text.

function init() {
    var composition = document.getElementById('composition');
    composition.addEventHandler('compositionupdate', onCompositionUpdate, false);
<body onload="init();">
<div id="composition"></div>

2. Background: What’s an Input Method Editor?

This section is non-normative.

An IME (input-method editor) is an application that allows a standard keyboard (such as a US-101 keyboard) to be used to type characters and symbols that are not directly represented on the keyboard itself. In China, Japan, and Korea, IMEs are used ubiquitously to enable standard keyboards to be employed to type the very large number of characters required for writing in Chinese, Japanese, and Korean.

On platforms with touch-based input device such as mobile phones, an IME also plays a role to type text that a simple on-screen keyboard cannot type directly.

A system IME is an IME already installed on a user's system.

An IME consists of two modules; a composer and a converter.

A composer is a context-free parser that composes non-ASCII characters (including phonetic characters) from keystrokes, e.g. Hiragana or Pinyin.

A converter is a context-sensitive parser that looks up a dictionary to convert phonetic characters to a set of ideographic characters, e.g. Kanji.

An IME clause is a grammatical word produced in an IME.

An IME selected clause is an IME clause currently being converted by an IME.

An IME composition is an instance of text produced in an IME. For IMEs that can produce multiple words, an IME composition consists of multiple IME clauses. For IMEs that produce only one word, an IME composition is equal to an IME clause.

When an IME receives keystrokes, it sends the keystrokes to a composer and receives phonetic characters matching to the keystrokes. When an IME receives phonetic characters from a composer, it sends the phonetic characters to a converter and receives the list of ideographic characters matching to the phonetic characters. The following figure shows the basic structure of an IME.

Fig. 1 Basic structure of an IME

2.1 Composer

This section is non-normative.

A composer consists of two types of composers: a phonetic composer and a radical composer.

A phonetic composer composes a phonetic character from its ASCII representation.

A radical composer composes a phonetic character from phonetic radicals.

A phonetic radical is a character component of a Latin character, a Chinese character, or a Korean character. A Latin character can consist of an ASCII character and accent marks, e.g. the character ‘á’ consists of the ASCII character ‘a’ and the accent mark ‘´’. A Chinese character can consist of Chinese character components that refer to its semantic origins, e.g. the Chinese character ‘略’ consists of two components ‘田’ and ‘各’. A Korean character consists of Korean character components that represent consonants or vowels, e.g. the Korean character ‘가’ consists of the consonant ‘ㄱ’ and the vowel ‘ㅏ’.

A composition window is a window that shows ASCII characters being composed by phonetic composers or phonetic radicals being composed by radical composers.

An IME usually shows the text being composed by a composer with its own style to distinguish it from the existing text. Even though most of composers output phonetic characters, some composers (such as Bopomofo composers) output a placeholder character instead of phonetic characters while composing text.

2.1.1 Phonetic Composer

This section is non-normative.

Phonetic composers are not only used for typing Simplified Chinese and Japanese, but also used for typing non-ASCII characters (such as mathematical symbols, Yi, Amharic, etc.) with a US-101 keyboard. Each of these languages has a mapping table from its character to a sequence of ASCII characters representing its pronunciation: e.g., ‘か’ to ‘ka’ in Japanese, and; ‘卡’ to ‘ka’ in Simplified Chinese. This mapping table is called as Romaji for Japanese and Pinyin for Simplified Chinese, respectively. A phonetic composer uses these mapping tables to compose a phonetic character from a sequence of ASCII characters produced by a US keyboard.

An example of a phonetic composer for Simplified Chinese outputs the ASCII characters that were input by the user, as its composition text.

Fig. 2 Composition text (Simplified Chinese)

On the other hand, a typical phonetic composer for Japanese outputs phonetic characters when the typed ASCII characters have corresponding phonetic characters.

Fig. 3 Composition text (Japanese)

An example of a phonetic composer for mathematical symbols outputs composed mathematical symbol and shows the source keystrokes in its own window, which is an example of a composition window.

Fig. 4 Composition text (Latex input)

2.1.2 Radical Composer

Radical composers are mainly used for typing Traditional Chinese and Korean with phonetic keyboards. Each phonetic keyboard of these languages can produce phonetic radicals: e.g., typing ‘r’ produces ‘ㄱ’ on a Korean keyboard; typing ‘o’ produces ‘人’ on a Traditional-Chinese (or Bopomofo) keyboard, etc. A radical composer composes a phonetic character from phonetic radicals given by these keyboards: e.g., typing ‘ㄱ’ (r) and ‘ㅏ’ (k) produces ‘가’ on a Korean keyboard; typing ‘人’ (o), ‘弓’ (n), and ‘火’ (f) produces ‘你’ on a Traditional-Chinese keyboard, etc.

A radical composer for Korean outputs the phonetic radicals as its composition text.

Fig. 5 Radical composer (Korean)

A radical composer for Traditional Chinese outputs a placeholder character (U+3000) and shows the phonetic radicals being composed to its own window. This window is an example of a composition window.

Fig. 6 Radical composer (Traditional Chinese)

Some platforms (such as Mac and Linux) use radical composers for typing accented characters used in European countries. For example, typing ‘ ̈ ’ (option+u) and ‘a’ (a) produces ‘ä’ on US keyboards of Mac.

Fig. 7 Radical composer (Mac)

2.1.3 On-Screen Keyboard

On touch-based platforms without hardware keyboard like mobile phone or tablet platforms, some kind of on-screen keyboard is displayed to help a user typing text, which occupies some part of the screen. A user uses this keyboard to type composition text.

Fig. 8 An example of an on-screen keyboard (English)

The layout of an on-screen keyboard may vary depending on language or its input modality (e.g. a telephone number input field requires number buttons only).

Fig. 9 An example of an on-screen keyboard (Japanese)

2.2 Converter

A converter is a context-sensitive parser used for replacing the outputs of a composer to ideographic characters on Chinese, Japanese, and Korean.


Korean seldom uses ideographic characters.

Because Chinese, Japanese, and Korean have many homonyms, each sequence of phonetic characters usually matches many ideographic characters: e.g., a Japanese phonetic character ‘か’ matches Japanese ideographic characters ‘化’, ‘科’, ‘課’, etc.; Pinyin characters ‘ka’ matches Simplified-Chinese ideographic characters ‘卡’, ‘喀’, ‘咯’, etc.; Bopomofo characters ‘人弓’ matches Traditional-Chinese ideographic characters ‘乞’, ‘亿’, ‘亇’, etc.

A converter looks up a dictionary and shows a list of candidates of possible ideographic characters so a user can choose one. This list is known as a candidate list. A candidate list is known as a candidate window when it has its own window.

Some Japanese IMEs show annotations in its candidate window for a character that is not so easy to distinguish from other characters (such as full-width alphabets, full-width Katakanas, and half-width Katakanas, etc.), as shown in the following figure.

Fig. 10 Candidate window (Japanese)

The next figure shows a candidate window of a Simplified-Chinese IME.

Fig. 11 Candidate window (Simplified Chinese)

And the next figure shows a candidate window of a Traditional-Chinese IME.

Fig. 12 Candidate window (Traditional Chinese)

Some techniques are used to improve conversion quality. For example, a converter integrates an MRU (Most-Recently Used) list. Even though there are many ideographic characters for each phonetic character (or phonetic radical), a user does not usually use all these ideographic characters. A converter uses an MRU list to filter out ideographic characters not used so often from a candidate list. Another example is a grammar parser. A converter that integrates a grammar parser splits the given phonetic characters into grammatical clauses and converts only one clause at a time. When a sequence of phonetic characters consists of n clauses and the i-th clause has m_i candidates, the total number of the candidates for the input characters becomes (m_1 * m_2 * … * m_n). To reduce the number of candidates owned by a converter, a converter usually processes one clause at a time. This clause is called the selected clause.

An IME usually renders a selected clause with a special style to distinguish it from other clauses, as shown in the following figure.

Fig. 13 Selected clause (Japanese)

When a converter converts two or more clauses, it chooses candidates for the selected clause so it becomes grammatically consistent with the surrounding clauses: e.g., Japanese converters usually output ‘危機一髪’ (not ‘危機一発’) for Japanese phonetic characters ‘ききいっぱつ’ because ‘危機一発’ is grammatically incorrect.

On a mobile platform, candidates may not appear in a separate window, but occupies some part of the screen for the user to choose the candidate word that they intend.

Fig. 14 Composition on mobile platform (Japanese)

Fig. 15 Composition on mobile platform (Japanese)

3. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this specification are to be interpreted as described in [RFC2119].

More to be written.

4. Terminology and Algorithms

To be written.

5. The inputMethodContext property

Authors can get an object which implements the InputMethodContext interface on an editable or focusable element, which can get keyboard input first (i.e. a “target” element). . If this property is accessed on non-editable, non-focusable element, a user agent SHOULD return a context for the innermost editable or focusable element to the element. In this case, the target property in the InputMethodContext interface points to the target element. If all ancestors of an element are neither editable nor focusable, this property returns null.

To control the IME attached to an element, it is a good idea to add this property to the HTMLElement interface.

partial interface HTMLElement {
    readonly    attribute InputMethodContext? inputMethodContext;

5.1 Attributes

inputMethodContext of type InputMethodContext, readonly , nullable

Returns an InputMethodContext interface associated with this element.

To change the behavior of the IME associated with an element, authors MUST first obtain an InputMethodContext interface from the inputMethodContext property of the HTMLElement interface.

The returned InputMethodContext interface MAY not be directly associated to the element. An interface MAY be shared among elements under an innermost editable or focusable element. In that case, target property in the InputMethodContext interface points to the element which owns the interface.

6. The Composition Interface

This interface represents an ongoing IME composition. It provides attributes representing the text being composed by an IME and its associated information.


It might be a good idea to add this interface to CompositionEvent.

interface Composition {
    readonly    attribute DOMString text;
    readonly    attribute long      selectionStart;
    readonly    attribute long      selectionEnd;
    sequence<unsigned long> getSegments ();

6.1 Attributes

selectionEnd of type long, readonly

Represents the 0-based index to the next position of the last character of the selection in the current composition text. If there is no selection, selectionStart and selectionEnd MUST be the same value and they indicate the position of the caret.

For example, if “DEF” is selected in composition text “abcDEFghi”, selectionStart is 3 and selectionEnd is 6. If the caret is between “B” and “C” in composition text “ABCD”, both selectionStart and selectionEnd are 2.

selectionStart of type long, readonly

Represents the 0-based index to the first character of the selection in the current composition text.


An author can assume that selectionStart and selectionEnd boundary fit in with any of segments, and that selectionStart is equal or less than selectionEnd.

text of type DOMString, readonly
Represents the text being composed by an IME. The text attribute of this node is equal to the text attribute of a compositionupdate event when this is referenced in compositionupdate's event handler.

6.2 Methods

If composition text is segmented into clauses by an IME, this array contains 0-based indices of the starting character of each clause in increasing order. If composition text is not segmented, it contains one '0' element.

This can be used to render compostion text as segmented clauses, but how to visualize them are not specified here. It is recommended to follow the platform standard way to visualize the segmentation (e.g. using underline or reverse background etc.).


The way the platform standard renders composition text might be customized by a user (e.g. for accessibility). CSS property to indicate the current default style to render the composition is desired.

No parameters.
Return type: sequence<unsigned long>

7. The InputMethodContext Interface

interface InputMethodContext {
    readonly    attribute Composition? composition;
    readonly    attribute DOMString    locale;
    readonly    attribute HTMLElement? target;
    void confirmComposition ();
    void setCaretRectangle (Node anchor, long x, long y, long w, long h);
    void setExclusionRectangle (Node anchor, long x, long y, long w, long h);

7.1 Attributes

composition of type Composition, readonly , nullable
Represents the current composition. If there is no ongoing composition, returns null.
locale of type DOMString, readonly
Represents the locale of the current input method as a BCP-47 tag (e.g. "en-US"). The locale MAY be the empty string when inapplicable or unknown.
target of type HTMLElement, readonly , nullable

Represents the element associated with the InputMethodContext object.

Once a target element gets deleted or modified not to accept any input, any access to the InputMethodContext interface through the object has no effect. Any method calls will just return, accesses to composition and target will return null, and accesses to locale will return the empty string.

7.2 Methods


Finishes the ongoing composition of the hosting user agent.

When a Web application calls this function, a user agent sends a compositionend event and a textInput event as a user types an ‘Accept’ key as written in “Input Method Editors” section the DOM Level 3 Events specification [DOM-LEVEL-3-EVENTS].


This may cause the race condition when a script calls confirmComposition while system IME gets accept key in the same time, a user agent may confirm the ongoing composition twice. Instead, introducing cancelComposition() is proposed and when a script needs the behavior of confirmComposition(), it can commit the current composition and then call cancelComposition().

No parameters.
Return type: void

Notifies the rectangle of composition text to a user agent. When a user agent renders a candidate window or a composition window, it uses this rectangle to draw windows next to the composition or prevent these windows from being rendered on this rectangle.

If a Web application wants to render composition text itself, it has to tell a user agent where the composition text will be located whenever it gets compositionstart event or compositionend event.

On Windows, this rectangle is used as a parameter for ImmSetCandidateWindow(). On Mac, this rectangle is sent when it calls [firstRectForCharacterRange:]. On Linux (GTK), this rectangle is used as a parameter for gtk_im_context_set_cursor_location().

  • The anchor parameter represents the DOM node against which the rectangle is positioned. This MAY be a different node than the node that listens to composition events which has focus and therefore a Web application can draw composition text where it does not have focus.
  • The x, and y are the offsets to the top-left of the rectangle relative to anchor node's top-left.
  • The w, and h are width and height of the rectangle.

These set values apply only to the current composition session (i.e. until compositionend event is sent).

A user agent MAY need to convert these coordinates to the screen coordinates when it shows a candidate window.

Return type: void

Gives a hint for a user agent to avoid showing any input related UI elements (e.g. on-screen keyboard, a candidate window) on the given rectangle, for an application to show some input-related UI elements (such as search suggestions) in the rectangle.

A user agent MAY use this hint to explicitly control the position for a candidate window, or determine zoom level and view port when on-screen keyboard comes in.

These set values apply only to the current composition session (i.e. until compositionend event is sent).

  • The anchor parameter represents the DOM node against which the rectangle is positioned.
  • The x, and y are the offsets to the top-left of the rectangle relative to anchor node's top-left.
  • The w, and h are width and height of the rectangle.
Return type: void

8. Best Practices

This section is non-normative.

This specification provides an interface for developing IME-aware Web applications.

This section describes practices for some use-cases.

8.1 Life of InputMethodContext

This section is non-normative.

Once a InputMethodContext interface is obtained, it should be valid for the lifetime of its target element's lifetime, as long as the element is editable or focusable. Once the target gets disabled, authors MAY NOT access an IME through the interface even after the target gets enabled again. Once the target is deleted, any access to the interface is void.

Any access to the InputMethodContext interface makes sense mostly when the target element has focus. In other words, it makes little sense if you access the interface when the target element doesn't have focus.

8.2 Drawing Composition Text


As of this writing, the order of compositionupdate event and DOM update is being discussed (see Bug 18931) and if compositionupdate event happens after DOM modification, this section has to be rewritten using beforeInput event, which is also being discussed. Example 2 code should be updated as well.

If a Web application wants to draw composition text by itself, it SHOULD handle the compositionupdate event to get notified from the IME that the composition text has been changed, and then use the interface described in this document to retrieve composition and let the IME know where the composition text is drawn by calling setCaretRectangle() method. If setCaretRectangle() is not called, IME will not have information about where to show IME UIs, and it may show UIs at an obtrusive position. To avoid this situation, a user agent may set some decent default position in the vicinity of the focused input field. Optionally it MAY call setExclusionRectangle() method to hint to the IME that a certain area is not suitable for showing the IME UI.

When a Web application draws composition text, it MUST call preventDefault() in compositionupdate handler so that the user agent will not draw the text.

When a Web application wants to handle DOM level3 composition events on a non-<input>, <textarea>, or contenteditable node, it MUST add inputmode attribute with a proper value and move focus to it to get composition events.

The following diagram shows the flow of events among the keyboard, the IME, the user agent, and the Web application when a user types ‘kyouha’ to convert to ‘今日は’.

Fig. 16 Event flow of IME and an IME-aware Web application.

A. Revision History

The following is an informative summary of the changes since the last publication of this specification. A complete revision history of the Editor's Drafts of this specification can be found here.

B. References

B.1 Normative references

Travis Leithead; Jacob Rossi; Doug Schepers; Björn Höhrmann; Philippe Le Hégaret. Document Object Model (DOM) Level 3 Events Specification. 6 September 2012. W3C Last Call Working Draft. URL: http://www.w3.org/TR/DOM-Level-3-Events/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt