You are going to have a hard time running JS Custom Elements or even the DOM on Mosaic! IE4 had an early DOM but it was different from the current one, and JS

You are going to have a hard time running JS Custom Elements or even the DOM on Mosaic! IE4 had an early DOM but it was different from the current one, and JS syntax was different too. Writing DOM-modifying code that does useful things on IE4 as well as on current browsers is possible, but requires a lot of extra code and testing for little gain, and will encounter browser bugs if you do anything complicated, so you can't just drop in a thoroughly-portable framework, you need to test as well. If what you mean is you'd like graceful fallback to a decent non-JS page on ancient browsers, that's doable, but then it doesn't matter if the JS framework only supports 99% of browsers. Just make sure to disable the framework on browsers too old to use it. (You'll still have a hard time with CSS on Mosaic.)