Validation and Accessibility
If your site isn't valid you aren't going to pass any accessibility tests. At the moment many designers dont see this as an issue, but with coming EU legislation it will become one for medium/large business (laws like these tend to only effect companies with X employees/turnover, i think). I'd strongly recommend that all your pages at least pass validation, in fairness its not that hard (I use the FireFox Web Developer plugin to check).
Of course if your client edits your code you have no control. You may be able to limit the damage by marking off, with comments, clear places in the html where they should edit (a news section for example) and by using CSS cleverly so they dont have to type many tags to enter a new item. So to add a new news item they should only have to add maybe a h2 tag and a 'p' tag and the text, defenatly no divs etc...
Option number two is to only allow them update through a CMS system, prob overkill for many sites but the only way to ensure the site stays valid. However a CMS system doesn't
guarantee an Accessible site and it'll take a good bit of work to get it there. (or you could hire me )
good luck
des