The Information Architecture will have been tested and finalised. From this, the complete site can be built, there are, however, a few more finalisation steps within the design.
There should be a wireframe prototype, a sitemap (which can be quite broad - this article describes the depth protyping can be taken), a navigational schema and some flat designs, a series of templates can built to W3C standards.
Ideally, any content is optimised for legibility and readability on the web as in this Jakob Nielsen article.
Practically, this means recognising that content for the web is less-easily read than content for print. Web users do not read articles whole, but skip from section to section. It aids usability to highlight key passages and summarise key points.
The site's pages will have been mocked-up from the grey-box prototypes in, for example, Photoshop or Fireworks. These pages then need to be laid out using XHTML/CSS.
The XHTML and CSS should be seperate – the XHTML should be able to stand on its own as a working page. The CSS can be used to alter the layout radically, changing the order, conditionally changing the context of the whole page.
This set of blueprints allows the developer to build the site, with no ambiguity as to how the site will work 'in practice'.
The vast majority of the work now needs to be carried out, with feedback and discussion between the client, designers and developer.
You may not keep everyone happy, or have created the most universal website in the world, but if the design process has been carried out effectively, you'll have a usable and extensible site, that has acheived the goals specified in the site requirements.