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diff --git a/ext/js/dom/sandbox/css-style-applier.js b/ext/js/dom/sandbox/css-style-applier.js
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+/*
+ * Copyright (C) 2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * This class is used to apply CSS styles to elements using a consistent method
+ * that is the same across different browsers.
+ */
+class CssStyleApplier {
+    /**
+     * Creates a new instance of the class.
+     * @param styleDataUrl The local URL to the JSON file continaing the style rules.
+     *   The style rules should be of the format:
+     *   [
+     *     {
+     *       selectors: [(selector:string)...],
+     *       styles: [
+     *         [(property:string), (value:string)]...
+     *       ]
+     *     }...
+     *   ]
+     */
+    constructor(styleDataUrl) {
+        this._styleDataUrl = styleDataUrl;
+        this._styleData = [];
+        this._cachedRules = new Map();
+    }
+
+    /**
+     * Loads the data file for use.
+     */
+    async prepare() {
+        this._styleData = await this._fetchJsonAsset(this._styleDataUrl);
+    }
+
+    /**
+     * Applies CSS styles directly to the "style" attribute using the "class" attribute.
+     * This only works for elements with a single class.
+     * @param elements An iterable collection of HTMLElement objects.
+     */
+    applyClassStyles(elements) {
+        const elementStyles = [];
+        for (const element of elements) {
+            const {className} = element;
+            if (className.length === 0) { continue; }
+            let cssTextNew = '';
+            for (const {selectorText, styles} of this._getRulesForClass(className)) {
+                if (!element.matches(selectorText)) { continue; }
+                cssTextNew += this._getCssText(styles);
+            }
+            cssTextNew += element.style.cssText;
+            elementStyles.push({element, style: cssTextNew});
+        }
+        for (const {element, style} of elementStyles) {
+            element.removeAttribute('class');
+            if (style.length > 0) {
+                element.setAttribute('style', style);
+            } else {
+                element.removeAttribute('style');
+            }
+        }
+    }
+
+    // Private
+
+    async _fetchJsonAsset(url) {
+        const response = await fetch(url, {
+            method: 'GET',
+            mode: 'no-cors',
+            cache: 'default',
+            credentials: 'omit',
+            redirect: 'follow',
+            referrerPolicy: 'no-referrer'
+        });
+        if (!response.ok) {
+            throw new Error(`Failed to fetch ${url}: ${response.status}`);
+        }
+        return await response.json();
+    }
+
+    _getRulesForClass(className) {
+        let rules = this._cachedRules.get(className);
+        if (typeof rules !== 'undefined') { return rules; }
+
+        rules = [];
+        this._cachedRules.set(className, rules);
+
+        const classNamePattern = new RegExp(`.${className}(?![0-9a-zA-Z-])`, '');
+        for (const {selectors, styles} of this._styleData) {
+            const selectorText = selectors.join(',');
+            if (!classNamePattern.test(selectorText)) { continue; }
+            rules.push({selectorText, styles});
+        }
+
+        return rules;
+    }
+
+    _getCssText(styles) {
+        let cssText = '';
+        for (const [property, value] of styles) {
+            cssText += `${property}:${value};`;
+        }
+        return cssText;
+    }
+}