I came across what appears to be a port to HTML5 of Paul Falstad's circuit simulator written in Java, so that it runs in the browser without a Java plugin. It seems to run smoother than the Falstad version so this is what I'll be using going forwards. Here's a screenshot:
Here's the URL in case you missed it above: https://lushprojects.com/circuitjs/
Incidentally if you know of a converter from circuitjs files to a *spice netlist, let me know. That's what I was searching for when I stumbled upon this.