Hi,

You will have to modify your HTML to add watermark because the HTML is always rendered with a white background. You can add additional output on top of your HTML (they are usually called "rubber stamp" instead of "watermark" because they appear on top of the main content, not below the main content), but because the HTML has a solid white background, it is not possible for you to add watermark after the conversion. However if you modify your HTML, such as applying a background image to your HTML's body element, then they will be rendered correctly under the main content.

Thanks!