diff --git a/tools/isobuild/bundler.js b/tools/isobuild/bundler.js
index 2c3bef67ba..74c66c665d 100644
--- a/tools/isobuild/bundler.js
+++ b/tools/isobuild/bundler.js
@@ -1119,8 +1119,7 @@ class Target {
 
       // Takes a CssOutputResource and returns a string of minified CSS,
       // or null to indicate no minification occurred.
-      // TODO Cache result by resource hash?
-      minifyCssResource(resource) {
+      minifyCssResource: (resource) => {
         if (! minifiersByExt.css ||
             minifyMode === "development") {
           // Indicates the caller should use the original resource.data
@@ -1142,6 +1141,7 @@ class Target {
           arch: target.arch,
           minifier: minifiersByExt.css,
           minifyMode,
+          watchSet: this.watchSet
         }).map(file => file.contents("utf8")).join("\n");
       }
     });
@@ -1691,6 +1691,7 @@ class ClientTarget extends Target {
       arch: this.arch,
       minifier: minifierDef,
       minifyMode,
+      watchSet: this.watchSet
     });
   }
 
@@ -1900,10 +1901,11 @@ function minifyCssFiles (files, {
   arch,
   minifier,
   minifyMode,
+  watchSet
 }) {
   const inputHashesByCssFile = new Map;
   const sources = files.map(file => {
-    const cssFile = new CssFile(file, { arch });
+    const cssFile = new CssFile(file, { arch, watchSet });
     inputHashesByCssFile.set(cssFile, file.hash());
     return cssFile;
   });
diff --git a/tools/isobuild/minifier-plugin.js b/tools/isobuild/minifier-plugin.js
index 7a34efbf37..e7ffe294a9 100644
--- a/tools/isobuild/minifier-plugin.js
+++ b/tools/isobuild/minifier-plugin.js
@@ -1,4 +1,14 @@
 import buildmessage from '../utils/buildmessage.js';
+import {
+  readAndWatchFileWithHash,
+} from '../fs/watch';
+import {
+  optimisticReadFile,
+  optimisticHashOrNull,
+} from "../fs/optimistic";
+import {
+  convertToPosixPath
+} from '../fs/files';
 const buildPluginModule = require('./build-plugin.js');
 
 class InputFile extends buildPluginModule.InputFile {
@@ -7,6 +17,7 @@ class InputFile extends buildPluginModule.InputFile {
 
     this._source = source;
     this._arch = options.arch;
+    this._watchSet = options.watchSet;
     this._minifiedFiles = [];
   }
 
@@ -75,5 +86,24 @@ export class CssFile extends InputFile {
   addStylesheet(options) {
     this._minifiedFiles.push({ ...options });
   }
-}
 
+  readAndWatchFileWithHash(path, { cache } = {}) {
+    const filePath = convertToPosixPath(path);
+
+    if (cache) {
+      const hash = optimisticHashOrNull(filePath);
+      const contents = optimisticReadFile(filePath);
+      this._watchSet.addFile(filePath, hash);
+
+      return {
+        hash,
+        contents
+      }
+    }
+
+    return readAndWatchFileWithHash(
+      this._watchSet,
+      filePath,
+    );
+  }
+}