these questions are a bit like asking "how do you lose weight"... everyones experience is totally unique, based on their own network, their own sites, their own linking/backlinks etc... so you are not going to get a real meaningful answer... since there are many factors that are all inter-related and doing a few things right and a lot of things wrong can have no negative impact... or doing a few things wrong and a lot of things right can have a massive negative impact.
css/validation are most likely totally meaningless. i'm not sure how people arrived at the conclusion that google somehow got worse at reading html since 1998.... particularly when on page content plays a minor role these days. probably something that was mentioned in the patent that took on a life of its own.
there is no such thing as "seo hosting" - people use separate class c's for what MIGHT be a benefit... but Google still knows what domains you own. that's not something you can hide.
- keyword in page title
- anchor text of inbound links from external sites
- anchor text of inbound links from internal links
- total number of inbound links from external sites
- contextual relevance of the sites linking in
- keywords in the body of the site
- keyword in the domain (more so for less competitive phrases)
- keywords in the meta keywords/description (more important for msn/yahoo)