UPDATE: This library now supports .NET Standard as opposed to .NET Core.
I have recently updated the MarkdownSharpCore library to support .NET Core 2.2 and now .NET Standard. The source code for the library can be found on GitHub at https://github.com/jasongaylord/markdownsharp. You can grab the NuGet package here: https://www.nuget.org/packages/MarkdownSharpCore.
MarkdownSharpCore allows you to present Markdown in the browser as HTML. There are a few applications that go the other route including Html2Markdown.