makes it possible to place a non-rectangular graphic object against the background of a Web page. Background transparency, supported by GIF and PNG formats, preserves transparent pixels in the image. These pixels blend with the Web page background in a browser.
Background matting, supported by GIF, PNG, and JPEG formats, simulates transparency by filling or blending transparent pixels with a matte color which you choose to match the Web page background on which the image will be placed. Background matting works only if the Web page background will be a solid color, and if you know what that color will be.
