diff --git a/src/plugins/create-optimized-css.ts b/src/plugins/create-optimized-css.ts
index 7e2ccf4..08e92c9 100644
--- a/src/plugins/create-optimized-css.ts
+++ b/src/plugins/create-optimized-css.ts
@@ -104,6 +104,11 @@ export function createOptimizedCss(options: CreateOptimizedCssOptions) {
             }
           });
 
+          // Do not proceed if font is not IBM Plex.
+          if (!attributes["font-family"].startsWith("IBM Plex")) {
+            return;
+          }
+
           const is_mono =
             attributes["font-style"] === "normal" &&
             attributes["font-family"] === "IBM Plex Mono" &&
diff --git a/tests/__snapshots__/create-optimized-css.test.ts.snap b/tests/__snapshots__/create-optimized-css.test.ts.snap
index 19bec55..a7d055f 100644
--- a/tests/__snapshots__/create-optimized-css.test.ts.snap
+++ b/tests/__snapshots__/create-optimized-css.test.ts.snap
@@ -9,8 +9,13 @@ exports[`create-optimized-css removes unused selectors 1`] = `
       "
 `;
 
-exports[`create-optimized-css removes unused @font rules 1`] = `
+exports[`create-optimized-css removes unused IBM @font rules 1`] = `
 "
+    @font-face {
+      font-family: 'CustomFont';
+      src: url('path/to/custom-font.ttf');
+    }
+
     @font-face {
       font-display: auto;
       font-family: IBM Plex Mono;
@@ -36,6 +41,11 @@ exports[`create-optimized-css removes unused @font rules 1`] = `
 
 exports[`create-optimized-css preserves all IBM fonts 1`] = `
 "
+    @font-face {
+      font-family: 'CustomFont';
+      src: url('path/to/custom-font.ttf');
+    }
+
     @font-face {
       font-display: auto;
       font-family: IBM Plex Mono;
diff --git a/tests/create-optimized-css.test.ts b/tests/create-optimized-css.test.ts
index ec3e414..c1e53e1 100644
--- a/tests/create-optimized-css.test.ts
+++ b/tests/create-optimized-css.test.ts
@@ -17,6 +17,11 @@ describe("create-optimized-css", () => {
   });
 
   const font_rules = `
+    @font-face {
+      font-family: 'CustomFont';
+      src: url('path/to/custom-font.ttf');
+    }
+
     @font-face {
       font-display: auto;
       font-family: IBM Plex Mono;