I haven't played with "Save for Web" feature, but i'm pretty sure that the output html, except for the table markup and images, should also contain some css styles that define the display behavior of the whole page.
So, when importing the html into VS, make sure associated styles are transfered too.
Also don't forget And to ensure HTML validity - you can choose the conformance level of your web app in the web.config or project properties (or just in the html editor - set Target Schema to XHTML Transitional and you should be sure that the html you got from Photoshop will be validated properly.