Lao Layout Requirements

W3C Group Draft Note

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Richard Ishida (W3C)
GitHub w3c/sealreq (pull requests, new issue, open issues)


This document describes or points to requirements for the layout and presentation of text in languages that use the Lao script. The target audience is developers of Web standards and technologies, such as HTML, CSS, Mobile Web, Digital Publications, and Unicode, as well as implementers of web browsers, ebook readers, and other applications that need to render Lao text.

Status of This Document

This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at

This document describes the basic requirements for Lao script layout and text support on the Web and in eBooks. These requirements provide information for Web technologies such as CSS, HTML and digital publications about how to support users of Lao scripts. Currently the document focuses on Lao as used for the Lao language. The information here is developed in conjunction with a document that summarises gaps in support on the Web for Lao.

The editor's draft of this document is being developed by the Southeast Asian Layout Task Force, part of the W3C Internationalization Interest Group. It is published by the Internationalization Working Group. The end target for this document is a Working Group Note.

This document was published by the Internationalization Working Group as a Group Draft Note using the Note track.

Group Draft Notes are not endorsed by W3C nor its Members.

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.

The W3C Patent Policy does not carry any licensing requirements or commitments on this document.

This document is governed by the 03 November 2023 W3C Process Document.

1. Introduction

1.1 Contributors

The initial information in this document was provided by Richard Ishida, drawing on the structure and information in Lao Orthography Notes.

Information and clarifications were provided by Anousak Anthony Souphavanh, Arthit Suriyawongkul, Ben Mitchell, James Clarke, John Durdin, Martin Hosken, and Norbert Lindenberg as members of the W3C's Southeast Asia Language Enablement community.

See also the GitHub contributors list for the Southeast Asia Language Enablement project, and the discussions related to Lao.

1.2 About this document

The aim of this document is to describe the basic requirements for Lao script layout and text support on the Web and in eBooks. These requirements provide information for Web technologies such as CSS, HTML and digital publications, and for application developers, about how to support users of the Lao script. The document currently focuses on texts using the Lao language.

The document focuses on typographic layout issues. For a deeper understanding of the Lao script itself and how it works see Lao Orthography Notes, which includes topics such as: Phonology, Vowels, Consonants, Encoding choices, and Numbers.

1.3 Gap analysis

This document is pointed to by a separate document, Lao Gap Analysis, which describes gaps in support for Lao on the Web, and prioritises and describes the impact of those gaps on the user.

Wherever an unsupported feature is indentified through the gap analysis process, the requirements for that feature need to be documented. The gap reports will typically point back to this document for more information.

As gaps in support for Lao are captured, the gaps can be brought to the attention of the relevant spec developer or the browser implementator community. The progress of such work is tracked in the Gap Analysis Pipeline.

This document should contain no reference to a particular technology. For example, it should not say "CSS does/doesn't do such and such", and it should not describe how a technology, such as CSS, should implement the requirements. It is technology agnostic, so that it will be evergreen, and it simply describes how the script works. The gap analysis document is the appropriate place for all kinds of technology-specific information.

To complement any content authored specifically for this document, the sections in the document also point to related, external information, tests, GitHub discussions, etc.

The Language enablement index points to this document and others, and provides a central location for developers and implementers to find information related to various scripts.

The W3C also has a repository with discussion threads related to the Lao script, including requests from developers to the user community for information about how scripts/languages work, and a notification system that tracks issues in W3C working groups related to Southeast Asian scripts. See a list of unresolved questions for Lao experts. Each section below points to related discussions. See also the repository home page.

2. Lao Script Overview

The Lao orthography is an alphabet. This means that both consonants and vowels are indicated.

Lao text runs left to right in horizontal lines. Spaces separate phrases, rather than words. There is no case distinction.

Each onset consonant is associated with a high, mid, or low class related to tone. Tone is indicated by a combination of the consonant class, the syllable type (live/dead), plus any tone mark.

No conjuncts are used for consonant clusters, except for one subjoined consonant, used in combination only with HA.

Syllable-initial clusters and syllable-final consonant sounds are all written with ordinary consonant letters. However, because all vowels are written, it is not difficult to algorithmically detect syllable boundaries.

Unlike its close relative, Thai, the Lao orthography is an alphabet and has no inherent vowel, but it still represents vowels using vowel signs (including combining marks, dedicated vowel letters, and a couple of repurposed consonants).

Vowels in Lao are written with a mixture of combining characters and ordinary spacing characters. Lao uses visual placement: only the vowel components that appear above or below the consonant are combining marks; the others are ordinary spacing characters that are typed in the order seen. There are pre-base vowel glyphs. In principle, there are no single-character circumgraphs in Lao text, but a single vowel or diphthong is frequently made up of multiple components.

There are no independent vowels, and standalone vowel sounds are written using vowel signs applied to U+0EAD LETTER O.

Lao has a large number of multipart vowels (including diphthongs) made from dedicated vowel characters and from consonants. Composite vowels can involve up to 4 glyphs (plus a tone mark), and glyphs can surround the base consonant(s) on up to 3 sides.

Vowels are often written differently when they appear in a closed vs. open syllable.

Lao has native digits, and they are commonly used.

3. Text direction

Lao is written horizontally, left to right.

4. Glyph shaping & positioning

4.1 Fonts & font styles

4.2 Context-based shaping & positioning

4.3 Letterform slopes, weights, & italics

5. Typographic units

5.1 Characters & encoding

5.2 Grapheme/word segmentation & selection

6. Punctuation & inline features

6.1 Phrase & section boundaries

6.2 Quotations & citations

6.3 Emphasis & highlighting

6.4 Abbreviation, ellipsis & repetition

6.5 Inline notes & annotations

6.6 Other inline features

7. Line & paragraph layout

7.1 Line breaking & hyphenation

Issue 1

Also "Some writers are beginning to use optional hyphens for syllable boundaries within words, which helps readability at line breaks." (@jmdurdin)

Issue 2

"One other difference from Thai is that European style punctuation (period, comma) is much more widely used in Lao than in Thai, with the consequence that traditional spaced phrase punctuation is now often incorrectly used, with spaces sometimes inserted within words." (@jmdurdin)

7.2 Text alignment & justification

Justification in Lao adjusts the space between phrases, but also makes certain adjustments to inter-character spacing.

7.3 Text spacing

7.4 Baselines, line height, etc.

7.5 Lists, counters, etc.

7.6 Styling initials

8. Page & book layout