Just a couple of thoughts. I work on a code base that was outsourced to an offshore company many years ago. I am pretty much guaranteed a few years worth of work to re-implement all of it. Your mileage may vary. The biggest mistake that was made when outsourcing was not demanding adequate code documentation. At some point you are going to want to update/alter/implement new features. If the person you hire to do updates or maintenance can't figure out what is going on, count on spending big bucks.
'Standards compliant code' is not realistic (or, I would argue, even a meaningful term). No two browsers agree on what the standards are, or how they should be interpreted. And they don't want to break old websites, so there are always going to be hacks to get pages to work on all browsers, even modern browsers. I have code that works perfectly in IE and Firefox, but Chrome can't handle it. Fortunately for my purposes, I do not have to support anything other than IE or Firefox. And there is no guarantee that the code is going to work on future browsers, 'compliant' or not.
@cryptodan, can you clarify what you mean by "Site should be done via CSS"? If you know how to create a website without any HTML, I would love to learn. All I can do is use CSS to style my elements.
Some other points you might want to consider:
1. How will conflicts be resolved?
2. How much time will pass before they respond to your emails/question?
3. How thorough are your specifications? Does it include screen shots, work flows, use cases, etc? You cannot expect them to guess at what you want.
If you could, report back on your experience?