Add support for CSS links in generated HTML outputs

internal/config/validate_test.go

@@ -328,6 +328,24 @@ func publishTransformPolicyCases() []publishTransformPolicyCase {
 				Input:   "report.md",
 			}},
 		},
+		{
+			name:    "html only sidecar css href allowed",
+			publish: PublishPolicy{HTML: true},
+			transform: Transform{MarkdownToHTML: &MarkdownToHTML{
+				Enabled: true,
+				Mode:    TransformModeSidecar,
+				CssHref: "/assets/report.css",
+			}},
+		},
+		{
+			name:    "html only index css href allowed",
+			publish: PublishPolicy{HTML: true},
+			transform: Transform{MarkdownToHTML: &MarkdownToHTML{
+				Enabled: true,
+				Mode:    TransformModeIndex,
+				CssHref: "assets/report.css?v=20260614",
+			}},
+		},
 		{
 			name:    "source and html sidecar allowed",
 			publish: PublishPolicy{Source: true, HTML: true},
@@ -425,6 +443,16 @@ func publishTransformPolicyCases() []publishTransformPolicyCase {
 			}},
 			wantErr: true,
 		},
+		{
+			name:    "disabled markdown css href rejected",
+			publish: PublishPolicy{Source: true},
+			transform: Transform{MarkdownToHTML: &MarkdownToHTML{
+				Enabled: false,
+				Mode:    TransformModeSidecar,
+				CssHref: "/assets/report.css",
+			}},
+			wantErr: true,
+		},
 		{
 			name:    "disabled markdown wrong mode rejected",
 			publish: PublishPolicy{Source: true},
@@ -436,3 +464,44 @@ func publishTransformPolicyCases() []publishTransformPolicyCase {
 		},
 	}
 }
+
+func TestValidateCSSHref(t *testing.T) {
+	tests := []struct {
+		name    string
+		value   string
+		wantErr bool
+	}{
+		{name: "empty"},
+		{name: "root relative", value: "/assets/report.css"},
+		{name: "relative", value: "assets/report.css"},
+		{name: "parent relative", value: "../assets/report.css"},
+		{name: "query", value: "/assets/report.css?v=20260614"},
+		{name: "http", value: "http://example.com/report.css"},
+		{name: "https", value: "https://example.com/assets/report.css?v=1"},
+		{name: "javascript", value: "javascript:alert(1)", wantErr: true},
+		{name: "data", value: "data:text/css,body{}", wantErr: true},
+		{name: "file", value: "file:///tmp/report.css", wantErr: true},
+		{name: "scheme relative", value: "//example.com/report.css", wantErr: true},
+		{name: "userinfo", value: "https://user@example.com/report.css", wantErr: true},
+		{name: "fragment", value: "/assets/report.css#main", wantErr: true},
+		{name: "space", value: "/assets/report css", wantErr: true},
+		{name: "tab", value: "/assets/report\tcss", wantErr: true},
+		{name: "newline", value: "/assets/report\ncss", wantErr: true},
+		{name: "backslash", value: `assets\report.css`, wantErr: true},
+		{name: "less than", value: "/assets/<report>.css", wantErr: true},
+		{name: "double quote", value: `/assets/"report".css`, wantErr: true},
+		{name: "single quote", value: "/assets/'report'.css", wantErr: true},
+		{name: "query only", value: "?v=1", wantErr: true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateCSSHref(tt.value)
+			if tt.wantErr && err == nil {
+				t.Fatal("validateCSSHref() error = nil, want error")
+			}
+			if !tt.wantErr && err != nil {
+				t.Fatalf("validateCSSHref() error = %v", err)
+			}
+		})
+	}
+}