https://xem.github.io/minix86/src/disassembler.html
doesn't work for me, maybe by "modern" you mean a current Chrome or Firefox.
I tested it with QTweb, Chrome 43 and Opera (Presto), certainly not the most "mainstream" or "popular" or "latest" browsers around, still I find no real reason why the thingy shouldn't work with them.
Uncaught SyntaxError: Unexpected token =>helpers.js:10
Uncaught SyntaxError: Unexpected token =>demos.js:2
Basically the first two actual instructions are not parsed correctly.
In Opera (12.15) I have three errors:
Syntax error at line 10 while loading: expected expression, got '>' var to16 = (n, l) => { -------------------^ Linked script compilation helpers.js Syntax error at line 38 while loading: invalid character after numeric literal: 'b'. n = b & 0b1111; -------------^ Linked script compilation disassembler.js Syntax error at line 2 while loading: expected expression, got '>' hello.onclick = e => { -------------------^ Linked script compilation demos.js