Now I use the asp:RequiredFieldValidator controls and suddenly I noticed that the error messages were no longer in red.
I then compared the projects - no obvious .ccs changes or anything like that.
Then I compared the web.config files and noticed that:
pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID" validateRequest="false"
had changed to:
pages controlRenderingCompatibilityVersion="4.0" clientIDMode="AutoID" validateRequest="false"
This must have happened during the wizard that migrates the project.
Have a look at:
What's New in ASP.NET 4 and Visual Web Developer
where it mentions that validators no longer render inline color:Red styles.
Changing it back to "3.5" sorted it out.
Isn't red the international error colour anyway?