diff --git a/eslint.config.js b/eslint.config.js
index 9dbc538..777a87c 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -6,7 +6,11 @@ export default [
       globals: {
         node: true,
         es6: true,
-        URL: "readonly"
+        URL: "readonly",
+        fetch: "readonly",
+        AbortController: "readonly",
+        setTimeout: "readonly",
+        clearTimeout: "readonly"
       },
       parserOptions: {
         ecmaVersion: 'latest',
diff --git a/lib/harAnalyzer.js b/lib/harAnalyzer.js
index 089ef3f..6bf1784 100644
--- a/lib/harAnalyzer.js
+++ b/lib/harAnalyzer.js
@@ -20,7 +20,7 @@ export class HarAnalyzer {
         this.version = this.package.version;
     }
 
-    transform2SimplifiedData(harData, url) {
+    async transform2SimplifiedData(harData, url) {
         const data = {
             'url': url,
             'rules': this.config.rules,
@@ -37,12 +37,25 @@ export class HarAnalyzer {
 
         let reqIndex = 1;
 
+        // CSS resources whose response body is missing from the HAR (served from
+        // cache, 304 Not Modified, or recorded with an empty body). We refetch
+        // these below — otherwise the custom properties they define are absent
+        // from the linted CSS and every var() referencing them elsewhere is
+        // falsely reported as "Unknown custom property" (no-unknown-custom-properties).
+        const missingCssUrls = new Set();
+
         for (const entry of harData.entries) {
             const req = entry.request;
             const res = entry.response;
             const reqUrl = req.url;
 
+            const mimeType = (res && res.content && res.content.mimeType) || '';
+            const looksLikeCss = mimeType.includes('css') || /\.css(\?|#|$)/i.test(reqUrl);
+
             if (!res.content || !res.content.text || !res.content.mimeType || !res.content.size || res.content.size <= 0 || !res.status) {
+                if (looksLikeCss && /^https?:\/\//i.test(reqUrl)) {
+                    missingCssUrls.add(reqUrl);
+                }
                 continue;
             }
 
@@ -62,6 +75,46 @@ export class HarAnalyzer {
             reqIndex++;
         }
 
+        // Refetch the bodies of CSS files that were absent from the HAR, skipping
+        // any we already captured. Failures (network error, timeout, non-OK) are
+        // ignored so a single unreachable file never fails the whole CSS test.
+        const refetchUrls = [...missingCssUrls].filter(
+            (cssUrl) => !data['style-files'].some((o) => o.url === cssUrl)
+        );
+        const refetched = await Promise.all(
+            refetchUrls.map(async (cssUrl) => {
+                try {
+                    const controller = new AbortController();
+                    const timeout = setTimeout(() => controller.abort(), 10000);
+                    const response = await fetch(cssUrl, { signal: controller.signal });
+                    clearTimeout(timeout);
+                    if (!response.ok) {
+                        return null;
+                    }
+                    const text = await response.text();
+                    if (!text) {
+                        return null;
+                    }
+                    return { url: cssUrl, content: text };
+                } catch {
+                    return null;
+                }
+            })
+        );
+        for (const item of refetched) {
+            if (!item) {
+                continue;
+            }
+            const obj = {
+                'url': item.url,
+                'content': item.content,
+                'index': reqIndex
+            };
+            data['all-styles'].push(obj);
+            data['style-files'].push(obj);
+            reqIndex++;
+        }
+
         // Extract <style> elements from HTML content
         for (const htmlObj of data.htmls) {
             const dom = new JSDOM(htmlObj.content);
@@ -229,7 +282,7 @@ export class HarAnalyzer {
             this.groups[group] = {};
         }
 
-        const analyzedData = this.transform2SimplifiedData(harData, url);
+        const analyzedData = await this.transform2SimplifiedData(harData, url);
         if (!('analyzedData' in this.groups[group])) {
             this.groups[group]['analyzedData'] = []
         }