Hello @David Stocks ,
Welcome to Microsoft Q&A forum.
Please check following points:
- Make sure that you are using a .NET Framework project(not .NET/.NET Core/ASP .NET Core project).
- Make sure that you have clicked and opened a web form(a .aspx file)(@Viorel has mentioned above).
- Check if the CSS Properties option hides in "Other Windows" list(View => Other Windows => CSS Properties).
- Try to directly search for "CSS Properties" in the VS Search box.
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