Transparency support accross ALL browsers for HTML5 chroma-key videos

I work for a website development company. Many of the websites we create include green screen/chroma-key videos that have an alpha channel associated with them that provides transparency to parts of the video. These videos are created utilizing HTML5. Most web browsers recognize the alpha channel/transparency, but Internet Explorer 8 does not. IE 9 sometimes recognizes the alpha channel, and sometimes not. Our clients rely on us to develop websites that work uniformly over all browsers. We are looking for a solution that allows all versions of Internet Explorer to acknowledge the alpha channel of HTML5 videos to create transparency for our websites. What advice can you give me please?