diff --git a/pom.xml b/pom.xml
index 1e6b4d6d..feada252 100644
--- a/pom.xml
+++ b/pom.xml
@@ -27,6 +27,12 @@
       <artifactId>zip4j</artifactId>
       <version>2.11.6</version>
     </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <version>5.13.4</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>
@@ -54,6 +60,12 @@
 
       </plugin>
 
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <version>3.5.4</version>
+      </plugin>
+
       <plugin>
         <groupId>com.diffplug.spotless</groupId>
         <artifactId>spotless-maven-plugin</artifactId>
diff --git a/src/test/java/io/github/eisop/website/EisopSiteGeneratorTest.java b/src/test/java/io/github/eisop/website/EisopSiteGeneratorTest.java
new file mode 100644
index 00000000..859134c0
--- /dev/null
+++ b/src/test/java/io/github/eisop/website/EisopSiteGeneratorTest.java
@@ -0,0 +1,112 @@
+/* (C)2026 */
+package io.github.eisop.website;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+/** Tests for the parts of {@link EisopSiteGenerator} that do not need the network. */
+class EisopSiteGeneratorTest {
+
+    @Test
+    void readableDateFromGitHubTimestamp() {
+        assertEquals(
+                "April 26, 2026", EisopSiteGenerator.getReadableDate("2026-04-26T23:05:21Z"));
+        assertEquals(
+                "December 20, 2024", EisopSiteGenerator.getReadableDate("2024-12-20T23:39:45Z"));
+        assertEquals(
+                "January 31, 2025", EisopSiteGenerator.getReadableDate("2025-01-31T00:00:00Z"));
+    }
+
+    @Test
+    void themeShippedPageKeepsOnlyTheBody() {
+        String html =
+                "<!DOCTYPE html>\n<html lang=\"en\">\n<head><title>T</title></head>\n"
+                        + "<BODY class=\"x\">\n<p>Download: <a href=\"/cf/a.zip\">a.zip</a></p>\n"
+                        + "</Body>\n</html>\n";
+        assertEquals(
+                "---\nlayout: default\n---\n<p>Download: <a href=\"/cf/a.zip\">a.zip</a></p>\n",
+                EisopSiteGenerator.themeShippedPage(html));
+    }
+
+    @Test
+    void themeShippedPageWithoutBodyTagsKeepsEverything() {
+        assertEquals(
+                "---\nlayout: default\n---\n<p>text</p>\n",
+                EisopSiteGenerator.themeShippedPage("  <p>text</p>  "));
+    }
+
+    @Test
+    void themeShippedPageKeepsTheVersionMarker() {
+        // The release scripts of the Checker Framework read the current version from this marker.
+        String marker =
+                "<!-- checker-framework-zip-version -->checker-framework-1.2.3.zip"
+                        + "<!-- /checker-framework-zip-version -->";
+        String themed =
+                EisopSiteGenerator.themeShippedPage(
+                        "<html><body><a href=\"/cf/x.zip\">" + marker + "</a></body></html>");
+        assertTrue(themed.contains(marker), themed);
+    }
+
+    @Test
+    void templatesArePackaged() throws IOException {
+        String cf = EisopSiteGenerator.readTemplate("cf-template.md");
+        assertTrue(cf.contains("$LatestCheckerFrameworkReleaseZip"));
+        assertTrue(cf.contains("$LatestAnnotationFileUtilitiesReleaseDownloadLink"));
+        String afu = EisopSiteGenerator.readTemplate("/afu-template.md");
+        assertTrue(afu.contains("$LatestAnnotationFileUtilitiesReleaseZip"));
+    }
+
+    @Test
+    void missingTemplateIsAnError() {
+        assertThrows(IOException.class, () -> EisopSiteGenerator.readTemplate("no-such.md"));
+    }
+
+    @Test
+    void afuPlaceholdersAreFilledOnEveryReleasePage(@TempDir Path cfDir) throws IOException {
+        String page =
+                "[$LatestAnnotationFileUtilitiesReleaseZip]"
+                        + "($LatestAnnotationFileUtilitiesReleaseDownloadLink)"
+                        + " ($LatestAnnotationFileUtilitiesReleaseDate)\n";
+        String filled = "[afu-1.zip](/afu/afu-1.zip) (May 1, 2026)\n";
+        Path top = write(cfDir.resolve("index.md"), page);
+        Path release = write(cfDir.resolve("checker-framework-1/index.md"), page);
+        Path other = write(cfDir.resolve("checker-framework-1/other.md"), page);
+
+        for (int run = 0; run < 2; run++) {
+            // The second run checks that the substitution is idempotent.
+            EisopSiteGenerator.substituteAfuPlaceholders(
+                    cfDir.toFile(), "afu-1.zip", "/afu/afu-1.zip", "May 1, 2026");
+            assertEquals(filled, read(top));
+            assertEquals(filled, read(release));
+            assertEquals(page, read(other));
+        }
+    }
+
+    @Test
+    void afuPlaceholdersToleratesMissingPages(@TempDir Path cfDir) throws IOException {
+        Files.createDirectories(cfDir.resolve("checker-framework-1"));
+        EisopSiteGenerator.substituteAfuPlaceholders(
+                cfDir.toFile(), "afu-1.zip", "/afu/afu-1.zip", "May 1, 2026");
+        EisopSiteGenerator.substituteAfuPlaceholders(
+                new File(cfDir.toFile(), "missing"), "afu-1.zip", "/afu/afu-1.zip", "May 1, 2026");
+    }
+
+    private static Path write(Path file, String content) throws IOException {
+        Files.createDirectories(file.getParent());
+        return Files.writeString(file, content, StandardCharsets.UTF_8);
+    }
+
+    private static String read(Path file) throws IOException {
+        return Files.readString(file, StandardCharsets.UTF_8);
+    }
+}