Phoca Extensions are (X)HTML and CSS valid. But there are some reasons why sometimes some outputed code is not 100% valid:
1) Phoca doesn't use only own methods to render the output, so sometimes some external method renders the output in Phoca Extensions (e.g. in some links there are & instead of &)
2) There are SEO reasons. SEO experts say, search engine robots try to behave as people, it means that if people like the site, robots will like the site too.
So on some parts of the site Phoca adds non valid items into (X)HTML and CSS code. This has only one reason. The site should be displayed similar in most used browsers (as example we can take some non valid CSS attribute for IE browser, which can provide the same displaying of the site in IE browser like in most used standard browsers). If you remove this item, you can satisfy one machine called validator, but contrariwise you can cancel required displaying for thousands (millions) of people, who are using browser, for which this item was added.
It means, if people like the site, they visit this site and they refer to this site, then the SEO value of this site will be growing. But if the site is only 100% valid, the only one output you get for this site is: "No errors" on validator site. No more, no less.
(X)HTML and CSS validity is important but if you want to have your site 100% valid, just be aware and do not remove some non valid items which are added into the code on purpose.
3) Validators don't know some tags and attributes which are specific for some external solutions (social or statistics code added by e.g. Google, Facebook, Twitter, etc.)
4) Validators mostly don't know new methods used in new browsers which are specific and help to work with new features.