Hi Laurent,
Is this only happening with a particular CSS file or with all CSS files? If its a specific file that is causing this issue, you can email it to me at omark-at-microsoft-dot-com, and I will have someone investigate.
You can also submit the bug via our online bug submission website at:
http://lab.msdn.microsoft.com/ProductFeedback/
The benefit of submitting the issue through the online website is that you can track status on it and can see the resolution for the bug. If you submit the bug via that site, please feel free to email me the bug ID and I'll follow up to make sure it gets priority.
Thanks.
Omar Khan
Visual Studio Web Tools