MediaWiki currently still supports browsers with ES5, but it seems the ES6 keyword const has slipped into the code of page previews.
"./src/constants.js": function(e, t, n) { "use strict"; var i = jQuery, r = i.bracketedDevicePixelRatio && i.bracketedDevicePixelRatio() || 1; const o = { off: "off", on: "on", control: "control" }; t.a = o, t.b = { THUMBNAIL_SIZE: 300 * r, EXTRACT_LENGTH: 525 } },
This causes fatal errors on older versions of Safari and IE.