I switched from MathJax to a middleman + execjs + KaTeX pipeline for my static sites earlier this year, and I'm very happy with it overall. KaTeX isn't 1-1 with MathJax when it comes to syntax support; I was surprised to see that underset and overset - which are ubiquitous - aren't supported. That said, I was able to work around every issue with alternative syntax. It's awesome that the heavy lifting is no longer client side, and it's a significantly easier build pipeline than MathJax if you're trying to generate a single minimized script or use webpack.