Comments inline:

>1. I gather that FlowLayout is preferred to GridLayout, especially for downlevel browsers. If so, it seems necessary to use tables to get a good looking layout for a data entry form. Is this true?

Yes, I have found it necessary to use HTML tables to get good results. I have seen several posts on other threads that indicate you can achieve the same results with CSS, but I have not tried it.

>2. A client has provided me with a fairly sophisticated CSS file which they'd like to see used as much as possible. I've included the .css in the project, but its styles don't seem to appear in the .style property sheet dropdowns for various controls/elements. Is VS capable of integrating the styles in this manner? Or, do I have to hand-code style info into the raw HTML?

You have to add the style sheet into the HTML for each .aspx page. In the head section, add the following line:

   <link href="MyStyleSheet.css" type="text/css" rel="stylesheet">

After you have linked the style sheet to your .apsx page, right-click on a web control and select Properties. In the Properties box, you'll see a field named CssClass. Enter the name of a class from your style sheet.

>3. In general, are CSSs well-supported in ASP.NET? Any tips or "best practices" on their use?

I'm not sure about that.