HTML/CSS is the web, so if you want to develop web apps, you have to learn it/use it. To enable any meaningful front-end interactivity, JavaScript is required as well. But with Microsoft Blazor (https://dotnet.microsoft.com/apps/aspnet/web-apps/blazor) there are starting to be alternative approaches via WebAssembly that allow you to use C# or other languages in lieu of JavaScript. If you don't like the flexibility of Flask, there is always Django (https://www.djangoproject.com).