If you are using Visual Studio 2005 and are experiencing random crashes when saving ASPX or HTML files with linked stylesheets, try this hotfix that we published recenty: Crash in vswebdesign.dll!CStyleSheet::LoadFromURL using Visual Studio 2005 SP1:
https://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=15541&wa=wsignin1.0

The hotfix applies to both VS 2005 and VWD 2005 Express. It does not apply to VS 2008 or VWD 2008 Express since they employ different HTML designer.