For example, HTML formatted with tables on browsers that do not support tables, look like a big paragraph of text with no particular formating. With frames, the problem is worse because the browser does not know how to obtain the files that are referenced within the frames. Some extentions like Java Script use the extentions of HTML in non-standard ways. According the the HTML 2.0 Specification, if a browser does not implement a tag, it is supposed to display the contents of the tag. In the case of Java Script, the contents is the script itself which the user has no interest in having displayed.
Some browsers fail miserably when the HTML within a file is invalid.
Others display it by making assumptions. It is best to check your HTML with several browsers to be sure that the content is visable in a rasonable format with each. The W3C HTML Validation Service provides a form to allow you to submit your URL for proper syntax. Insuring that you have proper syntax will insure that the maximum number of people will be able to view your pages properly. Some browsers may allow symantic errors that you may not notice while other browsers will simply fail to load a page because of a syntax error.