Document Source available from your browser's menu bar. As the name suggests, this allows you see the way an author has marked up their page. It is essential to check your HTML tagging on a browser before uploading, preferably on different machines. Aim to produce 'device independent' HTML, avoiding tags, code or features that are specific to one specific browser or platform. Try to avoid two opening body tags on the same page.
Frames have both advantages and disadvantages.
They are better suited to some purposes than others. We advise caution in their use. The web is always changing. Check any links regularly to make sure they are not 'dead'. These are frustrating for the user, who will click the link and wait for download, only to find no content.