While it is estimated that 80% of web surfers are using Netscape or
Internet Explorer, which permit some extensions to the HTML standard, 
it isn't nessarily so for the folks who will be visiting your pages.  
Some enhancements do not impact the file's readability with Mosaic or 
even Lynx, but others (especially tables) become a complete mess if the 
browser doesn't support them.  You should offer a text-only or standard 
HTML page alternative for such visitors.

Also note that some authors will overdo it with the HTML extensions,
especially backgrounds.  Backgrounds should be kept as neutral as
possible.  A busy background will often make it difficult to read a 
page, and strong colors will also obscure the text.  Keep the background
light and simple.

Frames seem to be the latest fad for "Netscape Enhanced" pages.  But
very few pages with frames are set up for easy use.  One of the main
problems is moving backwards in a chain: most pages with frames do not
allow you to back up because each page must have a backward pointer.
And if an anchor points to a page outside the local site, there is no 
easy way to return to the local site without starting over from the 
beginning.  Some browsers do allow you to back up by right-clicking  
in the frame (PC) or clicking and holding in the frame (Mac), but most
casual users are probably not going to know that.