If you check it though what they're using XSLTProcessor for, it seems to be a fallback for an MSXML polyfill, e.g. when you search for "XSLTProcessor" here you can see it: view-source:http://discuz.turzx.com/assets/forum.js?v=3c534b8a
So in the case of Flarum their DOMParser using alternative would chime in, as that's an additional fallback to the MSXML / ActiveX using polyfill.
I was checking for XSLTProcessor out of curiosity, and most of the top sites seem to be all using Flarum as a software?
Found it a nice fun fact: https://chromestatus.com/metrics/feature/timeline/popularity...
If you check it though what they're using XSLTProcessor for, it seems to be a fallback for an MSXML polyfill, e.g. when you search for "XSLTProcessor" here you can see it: view-source:http://discuz.turzx.com/assets/forum.js?v=3c534b8a
So in the case of Flarum their DOMParser using alternative would chime in, as that's an additional fallback to the MSXML / ActiveX using polyfill.