Web Content Accessibility Guideline (WCAG) proposed by Web Accessibility Initiative is the most recognized regulation in the world in evaluating the accessibility of web contents and is one of the W3Cdocumentations. Currently,the WCAG documents are maintained as a non-computable dictionary like resources. In this study, we proposed a methodology to develop an ontological knowledge base with rules for the WCAG 2.0. Two WCAG techniques in different groups of techniques are used to illustrate the creation of the knowledge base and their application programming interfaces. For demonstrating purpose, a web-based WCAG validation system is built. With the proposed knowledge base and interfaces, computer programs can decide whether certain web content satisfies a particular WCAG technique. In other words, the WCAG documents have been successfully trans-formed to a computable recourse. Such sharable knowledge base and programming interfaces can be embedded into any system requiring the WCAG knowledge.