The easiest way to handle all of this is to start now instead of waiting for it to be required by new browsers. If your using a database driven site and your templates are properly coded, it won't take much work to change the rendering of the stored content.
Here are some rough guidelines for HTML authors. If you use these, you are more likely to end up with pages that are easy to maintain, look acceptable to users regardless of the browser they are using, and can be accessed by the many Web users with disabilities. Meanwhile W3C have produced some more formal guidlines for authors. Have a look at the detailed accessibility guidelines.
[list=1][*]A question of style sheets. For most people the look of a document - the color, the font, the margins - are as important as the textual content of the document itself. But make no mistake! HTML is not designed to be used to control these aspects of document layout. What you should do is to use HTML to mark up headings, paragraphs, lists, hypertext links, and other structural parts of your document, and then add a style sheet to specify layout separately, just as you might do in a conventional Desk Top Publishing Package. That way, not only is there a better chance of all browsers displaying your document properly, but also, if you want to change such things as the font or color, it's really simple to do so. See the Touch of style.[*]FONT tag considered harmful! Many filters from word-processing packages, and also some HTML authoring tools, generate HTML code which is completely contrary to the design goals of the language. What they do is to look at a document almost purely from the point of view of layout, and then mimic that layout in HTML by doing tricks with FONT, BR and (non-breaking spaces). HTML documents are supposed to be structured around items such as paragraphs, headings and lists. Yet some of these documents barely have a paragraph tag in sight![*]The problem comes when the content of pages needs to be updated, or given a new layout, or re-cast in XML (which is now to be the new mark-up language). With proper use of HTML, such operations are not difficult, but with a muddle of non-structural tags it's quite a different matter; maintenance tasks become impractical. To correct pages suffering from injudicious use of FONT, try the HTML Tidy program, which will do its best to put things right and generate better and more manageable HTML.[*]Make your pages readable by those with disabilities. The Web is a tremendously useful tool for the visually impaired or blind user, but bear in mind that these users rely on speech synthesizers or Braille readers to render the text. Sloppy mark-up, or mark-up which doesn't have the layout defined in a separate style sheet, is hard for such software to deal with. Wherever possible, use a style sheet for the presentational aspects of your pages, using HTML purely for structural mark-up.[*]Also, remember to include descriptions with each image, and try to avoid server-side image maps. For tables, you should include a summary of the table's structure, and remember to associate table data with relevant headers. This will give non-visual browsers a chance to help orientate people as they move from one cell to the next. For forms, remember to include labels for form fields.