Web accessibility metrics are an invaluable tool for researchers, developers, governmental agencies and end users. Accessibility metrics help indicate the accessibility level of websites, including the accessibility level of individual websites, or even large-scale surveys of the accessibility of many websites. Recently, a plethora of metrics has been released to complement the A, AA, and AAA Levels measurement used by the WAI guidelines. However, the validity and reliability of most of these metrics are unknown and those making use of them are taking the risk of using inappropriate metrics. In order to address these concerns, this note provides a framework that considers validity, reliability, sensitivity, adequacy and complexity as the main qualities that a metric should have.