The postMessage API in HTML5 specification is useful for making cross domain calls across frames. This is typically useful for mashups, Web 2.0 sites (e.g., pageflakes.com) where different widgets might need to communicate with each other. Few developers have already started using HTML5 postMessage in their projects, without knowing why they are using. Here are a couple of questions an inquisitive developer might have in mind: 1. How are mashups and rich Web 2.0 applications built even before HTML5 postMessage API came into existence? 2. What is the trust model which Web 2.0 sites have? (Who trusts whom?) 3. Is there really a need for a new API when workarounds met the needs? This post tries to answer these questions and explains why postMessage API is important.