TLDR: Thanks to SharePoint, it is possible to write an Office UI Fabric styled button without using ReactJS or Office UI Fabric at all. The code is simple and easy to reproduce for any project. Copy the source this blog post contains and you are ready to go.
Create an Office UI Fabric button wihtout Office UI Fabric
The longer story is that recently, more theme slots are available in SharePoint Online than before. I created a reference chart for the documentation of SPFx-uifabric-theme project.
So isn’t it exciting to create buttons, hyperlinks and input fields just using SASS variables and fundamental HTML elements?