I am a bit obsess with error free validation and clean code, but I feel bad asking Varien provides prestine code as certainly, you guys have many more important things to do before the final release comes out..
On the other hand, you give us XHTML Strict doc-type. They are quite many validation errors in onepage checkout (haven’t work on other pages yet but expect to see errors too), many of them are easy to fix but not all as invalid attributes are used (e.g. ‘defaultValue’, ‘select’, ‘align’ etc...) , and the inline javascript also causing validation errors (missing <![CDATA[
... unescaped script content ...
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One or two maybe causing accessibility issue, for instance, the Address has two input fields yet only one ‘lable’ attribute is used that also causing ID conflict.