ASP.NET Client Side Validation Not Working for Entire App

I have an ASP.NET 4 web forms app that has been in production for almost a year. All of a sudden, on the production server only (IIS 7) client side validation is not firing. Server-side validation fires just fine, but nothing on the client side. There is no difference in the cod开发者_JS百科e between the two. I've compared the generated HTML/JavaScript and there are no major differences either.

Anybody experience this? Some strange IIS setting that my server admin didn't tell me about?