Did you try to embed some PHP code onto the page? There shouldn't be given it's a .html page, which shouldn't have any PHP. The error itself is somewhat typical of a BOM (byte order mark) issue, which relates to how the file was saved. Saving the file as utf-8 with no BOM usually fixes it. Considering your site is a iWeb one though it likely doesn't give that option. Are you editing the files after publishing at all or using some 3rd party software on it?