Convert em media queries to px

user-script.js

// ==UserScript==
// @name         px-media-queries
// @description  Convert all media queries to use px instead of em.
// @match        *://*/*
// @exclude      https://github.com/*
// @exclude      https://developer.mozilla.org/*
// @exclude      http://localhost:*/*
// ==/UserScript==
// Breaks Dark mode on Github for some reason.

(function() {
    'use strict';

    function updateSheet(sheet) {
        try {
            for (const rule of sheet.cssRules) {
                if (rule instanceof CSSMediaRule) {
                    rule.media.mediaText = rule.media.mediaText.replace(/(\d*(\.\d*)?)em/ig, (match, size) => {
                        return `${parseFloat(size) * 16}px`;
                    });
                }
            }
        } catch(_) {
        }
    }

    Object.values(document.styleSheets).forEach(sheet => {
        updateSheet(sheet);
    });

    const observer = new MutationObserver((mutationsList) => {
        for (let mutation of mutationsList) {
            if (mutation.type === 'childList') {
                const added = mutation.addedNodes;
                added.forEach((node) => {
                    if (node.tagName === 'LINK' && node.rel === 'stylesheet') {
                        updateSheet(node.sheet);
                    } else if (node.tagName === 'STYLE') {
                        updateSheet(node.sheet);
                    }
                });
            }
        }
    });

    observer.observe(document.documentElement, {
        childList: true,
        subtree: true
    });
})();