There are a number of things that you can do to ensure that your XHTML code will still display reasonably on earlier browsers.
Try to avoid the use of new tags (which do not work correctly with all browsers) for anything that is essential to the operation of your page or at least provide an alternate method of access to the same thing.
Nothing is worse than a page that displays nothing but a column of grey boxes because all of the links are Java Applets and the user has Java turned off (or doesn't have Java support at all). Come on, at least supply an alternate access method to some of the information.
Any image maps should also have alternate text links so that anyone with images turned off or using a text only browser can still use your site.
You need to cater for browsers that do not understand scripts or which have scripts disabled. You do this by placing some alternative text between <noscript> </noscript> tags.
The following optional attributes can be specified on the <noscript> tag:
The following tags have been deprecated (which means that they are not part of XHTML 1.1). You can still use these tags in your page if you use the transitional DOCTYPE. This will get you around some of the problems with new commands not being understood by current browsers. You should still try to minimize your use of these tags as they are no longer a part of the latest version of the standards.
This article written by Stephen Chapman, Felgall Pty Ltd.