In the .NET code for the HttpClient class I found this attribute: [UnsupportedOSPlatform("browser")]
So, several fields are not supported by the "browser" platform, but which platform would this be? WebAssembly? Windows? Anything else?
It's when your code runs within a browser. Right now that's Blazor WebAssembly (not Blazor Server, as the code actually runs on a server).