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About This Document

This document explains basic guidelines for writing a specification for the web platform. HTML, XMLHttpRequest, DOM Core, and others follow these guidelines.

See also Advice for people writing specs.


This document focuses primarily on specifications aimed at browser implementors. The content of these specifications focuses on API calls that browsers implement and the required browser behavior under all possible cases. Specifications for user interface design and for authored content are not addressed here.


Each specification includes these top-level sections:

  • Introduction — Gives an overview of the technology defined.
  • Conformance — References RFC 2119 and/or explains how conformance is determined in this specification. Lists the conformance classes that apply to this specification. See http://www.w3.org/TR/qaframe-spec/
  • Terminology — Defines where the terms in the specification originate from and sometimes provides a few novel definitions that do not fit within the main prose of the specification.
  • … content …
  • References
  • Acknowledgments

See e.g. Progress Events and XMLHttpRequest.


Content in a specification falls into two general categories: normative and informative. Normative content includes the requirements and definitions and uses verbs such as must, should, and may. If a normative statement uses the verb must (and applies to UAs), the browser implementor should be able to write a test case for it. (If it is not possible to write a test case for the statement, do not use the word must.) Informative content is everything that is not normative. Informative (non-normative) content includes diagrams and code examples—very useful content, but supportive in nature to the actual browser requirements.

See also Hixie's blog post on the subject: http://ln.hixie.ch/?start=1140242962&count=1


Elements, attributes, members of an object, algorithms, are all marked up using the dfn element. The title attribute of the element or its contents if there is no such attribute represents the name used for cross references. These are the conventions:

  • Interfaces: InterfaceName; e.g. interface Document { …
  • Interface members: dom-InterfaceName-attributeOrMethodName; e.g. the URL</code> attribute must …
  • Events: event-eventname
  • Elements: elementname
  • Attributes: attr-elementname-attributename
  • Concepts: concept-word; e.g. <dfn title=concept-node>node</dfn>

You reference a definition using either the span or code element. Anolis takes care of linking back to the definition.


APIs are defined using Web IDL. An interface for instance can be defined as follows:

<pre class=idl>interface <dfn>ProcessingInstruction</dfn> : <span>Node</span> {
  readonly attribute DOMString <span title=dom-ProcessingInstruction-target>target</span>;
           attribute DOMString <span title=dom-ProcessingInstruction-data>data</span>;

The name of the interface is within a dfn element whereas everything else is referenced as term and defined elsewhere. (See also ProcessingInstruction in DOM Core.)

The interface members are described both in a non-normative and a normative way. Non-normatively using a <dl class=domintro> construct. Normatively as described in the following sections. (See other specifications for examples.)


For each attribute, include the following content:

  • Purpose or general use of the attribute. Use the <dfn> tag for the definition.
  • What the browser must do when it retrieves the attribute ("on getting"). This is normative content.
  • What the browser must do when it writes a value to the attribute ("on setting"). This is normative content. This content must include requirements for setting the attribute to any value, not just to valid values. For example, the on setting description for an attribute that requires a float value must also prescribe what the browser does when the value is set to values of infinity, to zero, or to NaN (not a number), unless that is already defined in the IDL section.


For each method, include the following content:

  • Purpose or general use of the method. Use the dfn element for the definition.
  • What happens when the method is called. Include a list of numbered steps that describe what happens when the method is invoked. Be sure to cover every case. This list of steps helps the browser implementor ensure that every steps is covered and tested.

Use the var element for parameters and code element for method names.


Refer to the RFC2119 for the definitive use of verbs such as "must", "should", and "may". If you use the verb "must", the implementor must be able to write a test case for this requirement. The verb "may" merely grants permission and is used rarely. Do not use "may not." Do not use RFC2119 keywords in non-normative text such as notes, examples, or statements of facts (use words like "can" or "is" instead). Normal lowercase is preferred for readability.


The following references provide useful information about specification writing and tools to aid the process of writing and producing a specification.

  • Language and terminology: RFC2119. This document explains when and why to use those spec'y verbs like must, may, and should.
  • Interface definitions: Web IDL. This document defines an interface definition language (IDL) that is useful for describing interfaces intended to be implemented in web browsers. It defines how to write the IDL blocks, how to overload methods, and the basics of common JavaScript bindings. This standard format helps to streamline the definitions for common script objects and allows the spec to focus on defining the unique aspects of the requirements.
  • Formatting tool: Pimp My Spec. This irreverently named tool defines special markup elements that are processed by a Python script that adds the proper spec numbering, cross references, and table of contents. It's magic! This page demonstrates use of the prescribed markup elements.
  • Also useful: HTML Design Principles. It's always good to keep the Big Picture in mind.


Here are some links to sources that might be helpful in expanding the above: