Can you remember all the techniques and how to make your page ADA compliant? Well, I cannot. As a creator, I would rather spend my time thinking about the content than thinking about ADA compliance (as important as it is).
Yes, you can let a validator complain about non-compliance, and then fix those. But this is tedious!
A better approach is not to code your HTML documents by hand. Once you figure out the basic requirements of ADA compliance, automate the generation of HTML documents as much as possible!
There are many techniques to do this. CGI (common gateway interface) programming is one. You can code your webpages in PHP or Perl so that it is easier to update it for ADA compliance later on.
For more information about PHP or Perl CGI programming, please refer to CISW410 and CISW420.