How to Write a Specification
author: Josie Wernecke
status: DRAFT (25 June 2009)
About This Document
This document explains basic guidelines for writing a specification and is based on information provided by Ian Hickson, editor of the HTML 5 Specification.
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.
Organize the specification by classes. This is a reference document, not a teaching document, so the order should reflect the class hierarchy, with functional groupings of classes where appropriate.
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, 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.
The IDL section (a gray box in the HTML 5 Specification) provides the complete definition of all attributes, methods, and parameters for a given class. Use the IDL format defined in Web IDL.
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).
For each method, include the following content:
- Purpose or general use of the method. Use the <dfn> tag 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> tag for parameters and <code> tag 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."
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.
- 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.