fix: add missing hint for 'improve compatibility with browser fallbacks' article (#34728)

guide/english/certifications/responsive-web-design/basic-css/improve-compatibility-with-browser-fallbacks/index.md

@@ -0,0 +1,37 @@
+---
+title: Improve Compatibility with Browser Fallbacks
+---
+## Improve Compatibility with Browser Fallbacks
+
+<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+We need to add a fallback to the ```background``` property of the ```.black-box`` class.
+
+### Example
+
+```css
+  :root {
+    --black-color: black;
+  }
+  .black-box {
+    background: var(--black-color);
+    width: 100px;
+    height: 100px;
+  }
+```
+
+## Solution
+
+Add a fallback to the ```background``` property before the existing background declaration:
+
+```css
+  :root {
+    --black-color: black;
+  }
+  .black-box {
+    background: black;
+    background: var(--black-color);
+    width: 100px;
+    height: 100px;
+  }
+```
+