diff --git a/Dockerfile.builder b/Dockerfile.builder
index 596fe40..5fced20 100644
--- a/Dockerfile.builder
+++ b/Dockerfile.builder
@@ -32,7 +32,7 @@ RUN git clone --depth 1 https://github.com/AOMediaCodec/libavif.git && \
     cd /usr/local/bin && \
     ln -s /libavif/build/avifenc
 
-RUN npm install -g svgo uglify-js
+RUN npm install -g svgo uglify-js clean-css-cli
 
 COPY cmd/alt_path /cmd/alt_path
 RUN (cd /cmd/alt_path && go get && go build -o /usr/local/bin/alt_path)
diff --git a/compress.sh b/compress.sh
index 394052e..fa586d1 100755
--- a/compress.sh
+++ b/compress.sh
@@ -164,6 +164,13 @@ JS_FILES=$(
         -printf '%P\n' \
 )
 
+CSS_FILES=$(
+    find . -type f \
+        -iname '*.css' \
+        -size +1 \
+        -printf '%P\n' \
+)
+
 COMPRESSIBLE_FILES=$(
     find . -type f \
         -not -iname '*.gif' \
@@ -308,6 +315,15 @@ minify_js() { FILE=$1
 
 foreach "$JS_FILES" minify_js
 
+## minify css files
+
+minify_css() { FILE=$1
+    cleancss -o "$FILE.cleancss" -- "$FILE" || rm -f "$FILE.cleancss"
+    mv -f "$FILE.cleancss" "$FILE"
+}
+
+foreach "$CSS_FILES" minify_css
+
 ## precompress uncompressed files
 
 wait # some files that we need to compress may still be being worked on