Add HTML status page generation (v1.3.0)v1.3.0

- Add configurable HTML status page generation after each check-run
- Default output: /var/www/htdocs/buetow.org/self/gogios/index.html
- New config options: HTMLStatusFile (path) and HTMLDisable (bool)
- Auto-creates output directory if it doesn't exist
- Uses atomic writes (tmp file + rename) to prevent corruption
- Auto-refresh every 5 minutes via meta tag
- Minimal, clean styling based on f3s_fallback template
- W3C HTML5 compliant output
- Email notifications on I/O errors via existing notifyError mechanism
- Mirrors email report structure (status changes, unhandled alerts, stale alerts)
- Comprehensive unit tests including W3C compliance validation
- All user-generated content properly HTML-escaped for security

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

5 files changed, 801 insertions, 1 deletions

diff --git a/cmd/gogios/main.go b/cmd/gogios/main.go
index d223eb6..580d150 100644
--- a/cmd/gogios/main.go
+++ b/cmd/gogios/main.go
@@ -9,7 +9,7 @@ import (
 	"codeberg.org/snonux/gogios/internal"
 )
 
-const versionStr = "v1.2.1"
+const versionStr = "v1.3.0"
 
 func main() {
 	configFile := flag.String("cfg", "/etc/gogios.json", "The config file")
diff --git a/internal/config.go b/internal/config.go
index 740b19b..2ade802 100644
--- a/internal/config.go
+++ b/internal/config.go
@@ -14,6 +14,8 @@ type config struct {
 	SMTPServer       string `json:"SMTPServer,omitempty"`
 	SMTPDisable      bool   `json:"SMTPDisable,omitempty"` // TODO: Document this option
 	StateDir         string `json:"StateDir,omitempty"`
+	HTMLStatusFile   string `json:"HTMLStatusFile,omitempty"` // Path to HTML status file
+	HTMLDisable      bool   `json:"HTMLDisable,omitempty"`    // Disable HTML status page generation
 	CheckTimeoutS    int
 	CheckConcurrency int
 	StaleThreshold   int      `json:"StaleThreshold,omitempty"`
@@ -58,6 +60,11 @@ func newConfig(configFile string) (config, error) {
 		conf.StaleThreshold = 3600 // Default to 1 hour
 	}
 
+	if !conf.HTMLDisable && conf.HTMLStatusFile == "" {
+		conf.HTMLStatusFile = "/var/www/htdocs/buetow.org/self/gogios/index.html"
+		log.Println("Set HTMLStatusFile to " + conf.HTMLStatusFile)
+	}
+
 	return conf, nil
 }
 
diff --git a/internal/html.go b/internal/html.go
new file mode 100644
index 0000000..d453124
--- /dev/null
+++ b/internal/html.go
@@ -0,0 +1,290 @@
+package internal
+
+import (
+	"fmt"
+	"html"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// persistHTMLReport generates and persists the HTML status page.
+// Mirrors persistReport() pattern from run.go with atomic write.
+func persistHTMLReport(state state, subject string, conf config) error {
+	htmlFile := conf.HTMLStatusFile
+	htmlDir := filepath.Dir(htmlFile)
+
+	// Auto-create directory if it doesn't exist
+	// CLAUDE: Only create it when it doesnt exist yet
+	if err := os.MkdirAll(htmlDir, 0o755); err != nil {
+		return fmt.Errorf("failed to create directory %s: %w", htmlDir, err)
+	}
+
+	tmpFile := htmlFile + ".tmp"
+
+	f, err := os.Create(tmpFile)
+	if err != nil {
+		return fmt.Errorf("failed to create temp file: %w", err)
+	}
+	defer f.Close()
+
+	htmlContent := state.htmlReport(subject)
+	if _, err = f.WriteString(htmlContent); err != nil {
+		return fmt.Errorf("failed to write HTML: %w", err)
+	}
+
+	return os.Rename(tmpFile, htmlFile)
+}
+
+// htmlReport generates the complete HTML status page.
+// Mirrors state.report() pattern from state.go:133-163.
+func (s state) htmlReport(subject string) string {
+	var sb strings.Builder
+
+	// Calculate counts for header summary (without generating HTML yet)
+	numCriticals := s.countBy(func(cs checkState) bool {
+		return cs.Status == nagiosCritical
+	})
+	numWarnings := s.countBy(func(cs checkState) bool {
+		return cs.Status == nagiosWarning
+	})
+	numUnknown := s.countBy(func(cs checkState) bool {
+		return cs.Status == nagiosUnknown
+	})
+	numOK := s.countBy(func(cs checkState) bool {
+		return cs.Status == nagiosOk
+	})
+	numStale := s.countStale()
+
+	// Write HTML header with summary
+	sb.WriteString(htmlHeader(subject, numCriticals, numWarnings, numUnknown, numStale, numOK))
+
+	// Alerts with status changed section
+	sb.WriteString(`<div class="section">` + "\n")
+	sb.WriteString(`<h2>Alerts with status changed</h2>` + "\n")
+	changed := s.htmlReportChanged(&sb)
+	if !changed {
+		sb.WriteString(`<p>There were no status changes...</p>` + "\n")
+	}
+	sb.WriteString(`</div>` + "\n\n")
+
+	// Unhandled alerts section
+	sb.WriteString(`<div class="section">` + "\n")
+	sb.WriteString(`<h2>Unhandled alerts</h2>` + "\n")
+	hasUnhandled := (numCriticals + numWarnings + numUnknown) > 0
+	if hasUnhandled {
+		s.htmlReportUnhandledContent(&sb)
+	} else {
+		sb.WriteString(`<p>There are no unhandled alerts...</p>` + "\n")
+	}
+	sb.WriteString(`</div>` + "\n\n")
+
+	// Stale alerts section
+	sb.WriteString(`<div class="section">` + "\n")
+	sb.WriteString(`<h2>Stale alerts</h2>` + "\n")
+	if numStale == 0 {
+		sb.WriteString(`<p>There are no stale alerts...</p>` + "\n")
+	} else {
+		s.htmlReportStaleAlerts(&sb)
+	}
+	sb.WriteString(`</div>` + "\n\n")
+
+	sb.WriteString(htmlFooter())
+
+	return sb.String()
+}
+
+// htmlReportChanged generates HTML for checks with status changes.
+// Mirrors state.reportChanged() from state.go:166-192.
+func (s state) htmlReportChanged(sb *strings.Builder) (changed bool) {
+	if 0 < s.htmlReportBy(sb, true, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical && cs.changed()
+	}) {
+		changed = true
+	}
+
+	if 0 < s.htmlReportBy(sb, true, false, func(cs checkState) bool {
+		return cs.Status == nagiosWarning && cs.changed()
+	}) {
+		changed = true
+	}
+
+	if 0 < s.htmlReportBy(sb, true, false, func(cs checkState) bool {
+		return cs.Status == nagiosUnknown && cs.changed()
+	}) {
+		changed = true
+	}
+
+	if 0 < s.htmlReportBy(sb, true, false, func(cs checkState) bool {
+		return cs.Status == nagiosOk && cs.changed()
+	}) {
+		changed = true
+	}
+
+	return
+}
+
+// htmlReportUnhandledContent generates HTML content for unhandled alerts section.
+// Mirrors state.reportUnhandled() from state.go:194-214.
+func (s state) htmlReportUnhandledContent(sb *strings.Builder) {
+	s.htmlReportBy(sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical
+	})
+
+	s.htmlReportBy(sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosWarning
+	})
+
+	s.htmlReportBy(sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosUnknown
+	})
+}
+
+// htmlReportStaleAlerts generates HTML for stale checks.
+// Mirrors state.reportStaleAlerts() from state.go:216-220.
+func (s state) htmlReportStaleAlerts(sb *strings.Builder) int {
+	return s.htmlReportBy(sb, false, true, func(cs checkState) bool {
+		return cs.Epoch < s.staleEpoch
+	})
+}
+
+// htmlReportBy is the generic HTML generator for check items.
+// Mirrors state.reportBy() from state.go:222-262 but outputs HTML.
+func (s state) htmlReportBy(sb *strings.Builder, showStatusChange, isStaleReport bool,
+	filter func(cs checkState) bool,
+) (count int) {
+	for name, cs := range s.checks {
+		if !filter(cs) {
+			continue
+		}
+		if !isStaleReport && cs.Epoch < s.staleEpoch {
+			continue // skip stale checks in non-stale report
+		}
+		count++
+
+		sb.WriteString(`<div class="check-item">` + "\n")
+
+		// Show status change if applicable
+		if showStatusChange && cs.changed() {
+			sb.WriteString(htmlStatusBadge(nagiosCode(cs.PrevStatus)))
+			sb.WriteString(` <span class="arrow">→</span> `)
+		}
+
+		// Show current status
+		sb.WriteString(htmlStatusBadge(nagiosCode(cs.Status)))
+		sb.WriteString(": ")
+		sb.WriteString(html.EscapeString(name))
+		sb.WriteString(": ")
+		sb.WriteString(html.EscapeString(cs.output))
+
+		// Show federated source if applicable
+		if cs.federated() {
+			sb.WriteString(" [federated from ")
+			sb.WriteString(html.EscapeString(cs.federatedFrom))
+			sb.WriteString("]")
+		}
+
+		// Show stale duration if applicable
+		if isStaleReport {
+			lastCheckedAgo := time.Since(time.Unix(cs.Epoch, 0))
+			sb.WriteString(fmt.Sprintf(" (last checked %v ago)", lastCheckedAgo))
+		}
+
+		sb.WriteString("\n</div>\n")
+	}
+
+	return
+}
+
+// countStale counts the number of stale checks.
+// Helper function for generating summary counts.
+func (s state) countStale() int {
+	return s.countBy(func(cs checkState) bool {
+		return cs.Epoch < s.staleEpoch
+	})
+}
+
+// htmlHeader generates the HTML document header with embedded CSS and status summary.
+func htmlHeader(subject string, numCriticals, numWarnings, numUnknown, numStale, numOK int) string {
+	var sb strings.Builder
+
+	sb.WriteString(`<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <meta http-equiv="refresh" content="300">
+    <title>`)
+	sb.WriteString(html.EscapeString(subject))
+	sb.WriteString(`</title>
+    <style>
+        body {
+            font-family: sans-serif;
+            text-align: center;
+            padding-top: 50px;
+        }
+        .container {
+            max-width: 800px;
+            margin: 0 auto;
+        }
+        .summary {
+            margin: 20px 0;
+            font-weight: bold;
+        }
+        .section {
+            margin: 30px 0;
+            text-align: left;
+        }
+        .check-item {
+            margin: 10px 0;
+            padding: 5px;
+        }
+        .CRITICAL { color: #dc3545; }
+        .WARNING { color: #ff8c00; }
+        .UNKNOWN { color: #6c757d; }
+        .OK { color: #28a745; }
+        .footer {
+            margin-top: 40px;
+            font-size: 0.9em;
+            color: #666;
+        }
+    </style>
+</head>
+<body>
+    <div class="container">
+        <h1>Gogios Status Report</h1>
+        <div class="summary">C:`)
+	sb.WriteString(fmt.Sprintf("%d W:%d U:%d S:%d OK:%d", numCriticals, numWarnings, numUnknown, numStale, numOK))
+	sb.WriteString(`</div>
+        <p>Last Updated: `)
+	sb.WriteString(time.Now().Format("2006-01-02 15:04:05 MST"))
+	sb.WriteString(`</p>
+
+`)
+
+	return sb.String()
+}
+
+// htmlFooter generates the HTML document footer.
+func htmlFooter() string {
+	var sb strings.Builder
+
+	sb.WriteString(`        <div class="footer">
+            Generated by Gogios at `)
+	sb.WriteString(time.Now().Format("2006-01-02 15:04:05 MST"))
+	sb.WriteString(`
+        </div>
+    </div>
+</body>
+</html>
+`)
+
+	return sb.String()
+}
+
+// htmlStatusBadge generates a colored HTML span for a status code.
+func htmlStatusBadge(status nagiosCode) string {
+	statusStr := status.Str()
+	return fmt.Sprintf(`<span class="%s">%s</span>`, statusStr, statusStr)
+}
diff --git a/internal/html_test.go b/internal/html_test.go
new file mode 100644
index 0000000..b22eea4
--- /dev/null
+++ b/internal/html_test.go
@@ -0,0 +1,496 @@
+package internal
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// TestHtmlStatusBadge tests the htmlStatusBadge function.
+func TestHtmlStatusBadge(t *testing.T) {
+	tests := []struct {
+		status   nagiosCode
+		expected string
+	}{
+		{nagiosOk, `<span class="OK">OK</span>`},
+		{nagiosWarning, `<span class="WARNING">WARNING</span>`},
+		{nagiosCritical, `<span class="CRITICAL">CRITICAL</span>`},
+		{nagiosUnknown, `<span class="UNKNOWN">UNKNOWN</span>`},
+	}
+
+	for _, tt := range tests {
+		result := htmlStatusBadge(tt.status)
+		if result != tt.expected {
+			t.Errorf("htmlStatusBadge(%v) = %q, want %q", tt.status, result, tt.expected)
+		}
+	}
+}
+
+// TestHtmlHeader tests the htmlHeader function.
+func TestHtmlHeader(t *testing.T) {
+	subject := "GOGIOS Report [C:1 W:2 U:3 S:4 OK:5]"
+	result := htmlHeader(subject, 1, 2, 3, 4, 5)
+
+	// Check that the header contains expected elements
+	expectedElements := []string{
+		"<!DOCTYPE html>",
+		`<html lang="en">`,
+		"<head>",
+		`<meta charset="UTF-8">`,
+		`<meta name="viewport"`,
+		"<title>",
+		subject,
+		`<meta http-equiv="refresh" content="300">`,
+		"<style>",
+		".CRITICAL { color: #dc3545; }",
+		".WARNING { color: #ff8c00; }",
+		"Gogios Status Report",
+		"C:1 W:2 U:3 S:4 OK:5",
+		"Last Updated:",
+	}
+
+	for _, elem := range expectedElements {
+		if !strings.Contains(result, elem) {
+			t.Errorf("htmlHeader() missing expected element: %q", elem)
+		}
+	}
+}
+
+// TestHtmlFooter tests the htmlFooter function.
+func TestHtmlFooter(t *testing.T) {
+	result := htmlFooter()
+
+	expectedElements := []string{
+		"Generated by Gogios at",
+		"</div>",
+		"</body>",
+		"</html>",
+	}
+
+	for _, elem := range expectedElements {
+		if !strings.Contains(result, elem) {
+			t.Errorf("htmlFooter() missing expected element: %q", elem)
+		}
+	}
+}
+
+// TestHtmlReportBy tests the htmlReportBy method with various filters.
+func TestHtmlReportBy(t *testing.T) {
+	now := time.Now().Unix()
+	staleEpoch := now - 7200 // 2 hours ago
+
+	s := state{
+		staleEpoch: staleEpoch,
+		checks: map[string]checkState{
+			"check1": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Service is down",
+			},
+			"check2": {
+				Status:     nagiosWarning,
+				PrevStatus: nagiosWarning,
+				Epoch:      now,
+				output:     "High load",
+			},
+			"check3": {
+				Status:     nagiosOk,
+				PrevStatus: nagiosOk,
+				Epoch:      staleEpoch - 100, // stale
+				output:     "All good",
+			},
+		},
+	}
+
+	// Test critical filter
+	var sb strings.Builder
+	count := s.htmlReportBy(&sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical
+	})
+
+	if count != 1 {
+		t.Errorf("htmlReportBy(critical filter) count = %d, want 1", count)
+	}
+
+	result := sb.String()
+	if !strings.Contains(result, "check1") {
+		t.Error("htmlReportBy(critical filter) missing check1")
+	}
+	if !strings.Contains(result, "Service is down") {
+		t.Error("htmlReportBy(critical filter) missing output text")
+	}
+	if !strings.Contains(result, "CRITICAL") {
+		t.Error("htmlReportBy(critical filter) missing CRITICAL status")
+	}
+
+	// Test status change filter
+	sb.Reset()
+	count = s.htmlReportBy(&sb, true, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical && cs.changed()
+	})
+
+	if count != 1 {
+		t.Errorf("htmlReportBy(changed filter) count = %d, want 1", count)
+	}
+
+	result = sb.String()
+	if !strings.Contains(result, "→") {
+		t.Error("htmlReportBy(changed filter) missing status change arrow")
+	}
+	if !strings.Contains(result, "OK") {
+		t.Error("htmlReportBy(changed filter) missing previous OK status")
+	}
+}
+
+// TestHtmlReportChanged tests the htmlReportChanged method.
+func TestHtmlReportChanged(t *testing.T) {
+	now := time.Now().Unix()
+
+	s := state{
+		staleEpoch: now - 3600,
+		checks: map[string]checkState{
+			"check1": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Service failed",
+			},
+			"check2": {
+				Status:     nagiosOk,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Still OK",
+			},
+		},
+	}
+
+	var sb strings.Builder
+	changed := s.htmlReportChanged(&sb)
+
+	if !changed {
+		t.Error("htmlReportChanged() returned false, want true")
+	}
+
+	result := sb.String()
+	if !strings.Contains(result, "check1") {
+		t.Error("htmlReportChanged() missing changed check")
+	}
+	if strings.Contains(result, "check2") {
+		t.Error("htmlReportChanged() should not include unchanged check")
+	}
+}
+
+// TestHtmlReport tests the complete HTML report generation.
+func TestHtmlReport(t *testing.T) {
+	now := time.Now().Unix()
+	staleEpoch := now - 3600
+
+	s := state{
+		staleEpoch: staleEpoch,
+		checks: map[string]checkState{
+			"critical_check": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Service is down",
+			},
+			"warning_check": {
+				Status:     nagiosWarning,
+				PrevStatus: nagiosWarning,
+				Epoch:      now,
+				output:     "High CPU usage",
+			},
+			"ok_check": {
+				Status:     nagiosOk,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Everything fine",
+			},
+			"stale_check": {
+				Status:     nagiosOk,
+				PrevStatus: nagiosOk,
+				Epoch:      staleEpoch - 100,
+				output:     "Not updated recently",
+			},
+		},
+	}
+
+	subject := "GOGIOS Report [C:1 W:1 U:0 S:1 OK:2]"
+	result := s.htmlReport(subject)
+
+	// Check that all major sections are present
+	expectedSections := []string{
+		"<!DOCTYPE html>",
+		"Gogios Status Report",
+		"Alerts with status changed",
+		"Unhandled alerts",
+		"Stale alerts",
+		"Generated by Gogios",
+		"</html>",
+	}
+
+	for _, section := range expectedSections {
+		if !strings.Contains(result, section) {
+			t.Errorf("htmlReport() missing section: %q", section)
+		}
+	}
+
+	// Check that check names appear
+	if !strings.Contains(result, "critical_check") {
+		t.Error("htmlReport() missing critical_check")
+	}
+	if !strings.Contains(result, "warning_check") {
+		t.Error("htmlReport() missing warning_check")
+	}
+
+	// Check status summary
+	if !strings.Contains(result, "C:1 W:1 U:0 S:1 OK:2") {
+		t.Error("htmlReport() missing correct status summary")
+	}
+}
+
+// TestHtmlEscaping tests that HTML special characters are properly escaped.
+func TestHtmlEscaping(t *testing.T) {
+	now := time.Now().Unix()
+
+	s := state{
+		staleEpoch: now - 3600,
+		checks: map[string]checkState{
+			"<script>alert('xss')</script>": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Output with <tags> & \"quotes\"",
+			},
+		},
+	}
+
+	var sb strings.Builder
+	s.htmlReportBy(&sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical
+	})
+
+	result := sb.String()
+
+	// Check that HTML is escaped
+	if strings.Contains(result, "<script>") {
+		t.Error("htmlReportBy() did not escape <script> tag in check name")
+	}
+	if !strings.Contains(result, "&lt;script&gt;") {
+		t.Error("htmlReportBy() missing escaped <script> tag")
+	}
+	if strings.Contains(result, "<tags>") && !strings.Contains(result, "&lt;tags&gt;") {
+		t.Error("htmlReportBy() did not escape <tags> in output")
+	}
+}
+
+// TestPersistHTMLReport tests the file creation and atomic write.
+func TestPersistHTMLReport(t *testing.T) {
+	// Create a temporary directory for testing
+	tmpDir := t.TempDir()
+	htmlFile := filepath.Join(tmpDir, "subdir", "status.html")
+
+	conf := config{
+		HTMLStatusFile: htmlFile,
+	}
+
+	now := time.Now().Unix()
+	s := state{
+		staleEpoch: now - 3600,
+		checks: map[string]checkState{
+			"test_check": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Test output",
+			},
+		},
+	}
+
+	subject := "GOGIOS Report [C:1 W:0 U:0 S:0 OK:0]"
+
+	// Test that the function creates the directory and file
+	err := persistHTMLReport(s, subject, conf)
+	if err != nil {
+		t.Fatalf("persistHTMLReport() error = %v", err)
+	}
+
+	// Check that the file was created
+	if _, err := os.Stat(htmlFile); os.IsNotExist(err) {
+		t.Error("persistHTMLReport() did not create the HTML file")
+	}
+
+	// Check that the temp file was removed
+	tmpFile := htmlFile + ".tmp"
+	if _, err := os.Stat(tmpFile); !os.IsNotExist(err) {
+		t.Error("persistHTMLReport() did not clean up temporary file")
+	}
+
+	// Read and verify the content
+	content, err := os.ReadFile(htmlFile)
+	if err != nil {
+		t.Fatalf("Failed to read HTML file: %v", err)
+	}
+
+	contentStr := string(content)
+	if !strings.Contains(contentStr, "<!DOCTYPE html>") {
+		t.Error("HTML file missing DOCTYPE declaration")
+	}
+	if !strings.Contains(contentStr, "test_check") {
+		t.Error("HTML file missing test check")
+	}
+	if !strings.Contains(contentStr, "Test output") {
+		t.Error("HTML file missing check output")
+	}
+	if !strings.Contains(contentStr, "C:1 W:0 U:0 S:0 OK:0") {
+		t.Error("HTML file missing status summary")
+	}
+}
+
+// TestFederatedChecks tests HTML generation for federated checks.
+func TestFederatedChecks(t *testing.T) {
+	now := time.Now().Unix()
+
+	s := state{
+		staleEpoch: now - 3600,
+		checks: map[string]checkState{
+			"remote_check": {
+				Status:        nagiosCritical,
+				PrevStatus:    nagiosOk,
+				Epoch:         now,
+				output:        "Remote service down",
+				federatedFrom: "remote.example.com",
+			},
+		},
+	}
+
+	var sb strings.Builder
+	s.htmlReportBy(&sb, false, false, func(cs checkState) bool {
+		return cs.Status == nagiosCritical
+	})
+
+	result := sb.String()
+
+	if !strings.Contains(result, "federated from") {
+		t.Error("htmlReportBy() missing federated indicator")
+	}
+	if !strings.Contains(result, "remote.example.com") {
+		t.Error("htmlReportBy() missing federated hostname")
+	}
+}
+
+// TestW3CCompliance tests that generated HTML meets W3C HTML5 standards.
+func TestW3CCompliance(t *testing.T) {
+	now := time.Now().Unix()
+
+	s := state{
+		staleEpoch: now - 3600,
+		checks: map[string]checkState{
+			"test_check": {
+				Status:     nagiosCritical,
+				PrevStatus: nagiosOk,
+				Epoch:      now,
+				output:     "Test output",
+			},
+		},
+	}
+
+	subject := "GOGIOS Report [C:1 W:0 U:0 S:0 OK:0]"
+	html := s.htmlReport(subject)
+
+	// W3C HTML5 Required Elements
+	requiredElements := map[string]string{
+		"DOCTYPE":           "<!DOCTYPE html>",
+		"html with lang":    `<html lang="en">`,
+		"charset meta":      `<meta charset="UTF-8">`,
+		"viewport meta":     `<meta name="viewport"`,
+		"title":             "<title>",
+		"closing html":      "</html>",
+		"closing head":      "</head>",
+		"closing body":      "</body>",
+		"proper html close": "</html>",
+	}
+
+	for name, elem := range requiredElements {
+		if !strings.Contains(html, elem) {
+			t.Errorf("W3C compliance check failed: missing %s (%q)", name, elem)
+		}
+	}
+
+	// Check that DOCTYPE is at the very beginning
+	if !strings.HasPrefix(html, "<!DOCTYPE html>") {
+		t.Error("W3C compliance: DOCTYPE must be the first line")
+	}
+
+	// Check proper tag nesting (basic validation)
+	htmlTagStart := strings.Index(html, "<html")
+	htmlTagEnd := strings.LastIndex(html, "</html>")
+	if htmlTagStart == -1 || htmlTagEnd == -1 {
+		t.Error("W3C compliance: missing <html> or </html> tag")
+	}
+	if htmlTagStart > htmlTagEnd {
+		t.Error("W3C compliance: </html> appears before <html>")
+	}
+
+	headStart := strings.Index(html, "<head>")
+	headEnd := strings.Index(html, "</head>")
+	if headStart == -1 || headEnd == -1 {
+		t.Error("W3C compliance: missing <head> or </head> tag")
+	}
+	if headStart > headEnd {
+		t.Error("W3C compliance: </head> appears before <head>")
+	}
+
+	bodyStart := strings.Index(html, "<body>")
+	bodyEnd := strings.LastIndex(html, "</body>")
+	if bodyStart == -1 || bodyEnd == -1 {
+		t.Error("W3C compliance: missing <body> or </body> tag")
+	}
+	if bodyStart > bodyEnd {
+		t.Error("W3C compliance: </body> appears before <body>")
+	}
+
+	// Head should come before body
+	if headEnd > bodyStart {
+		t.Error("W3C compliance: <head> section should be closed before <body> starts")
+	}
+
+	// Check for proper character encoding in meta tag
+	if !strings.Contains(html, `charset="UTF-8"`) {
+		t.Error("W3C compliance: charset should be UTF-8")
+	}
+
+	// Verify no common HTML errors
+	commonErrors := []string{
+		"<<",     // Double tag opening
+		">>",     // Double tag closing
+		"< ",     // Space after tag opening
+		" >",     // Space before tag closing (in tag name)
+		"<//>",   // Malformed closing tag
+		"</ >",   // Empty closing tag
+		"< >",    // Empty tag
+	}
+
+	for _, err := range commonErrors {
+		if strings.Contains(html, err) {
+			t.Errorf("W3C compliance: found malformed HTML pattern: %q", err)
+		}
+	}
+
+	// Verify all div tags are properly closed
+	divOpen := strings.Count(html, "<div")
+	divClose := strings.Count(html, "</div>")
+	if divOpen != divClose {
+		t.Errorf("W3C compliance: mismatched div tags (open: %d, close: %d)", divOpen, divClose)
+	}
+
+	// Verify all span tags are properly closed
+	spanOpen := strings.Count(html, "<span")
+	spanClose := strings.Count(html, "</span>")
+	if spanOpen != spanClose {
+		t.Errorf("W3C compliance: mismatched span tags (open: %d, close: %d)", spanOpen, spanClose)
+	}
+}
diff --git a/internal/run.go b/internal/run.go
index 91fe323..eed8ad5 100644
--- a/internal/run.go
+++ b/internal/run.go
@@ -39,6 +39,13 @@ func Run(ctx context.Context, configFile string, renotify, force bool) {
 	if err := persistReport(subject, body, conf); err != nil {
 		notifyError(conf, err)
 	}
+
+	// Generate HTML status page (unless disabled)
+	if !conf.HTMLDisable {
+		if err := persistHTMLReport(state, subject, conf); err != nil {
+			notifyError(conf, err)
+		}
+	}
 }
 
 func persistReport(subject, body string, conf config) error {