Add support for CSS links in generated HTML outputs

internal/config/config.go

@@ -110,6 +110,7 @@ type MarkdownToHTML struct {
 	Enabled bool   `yaml:"enabled"`
 	Mode    string `yaml:"mode"`
 	Input   string `yaml:"input"`
+	CssHref string `yaml:"css_href"`
 }
 
 type PathMapping struct {

internal/config/validate.go

@@ -2,8 +2,10 @@ package config
 
 import (
 	"fmt"
+	"net/url"
 	"regexp"
 	"strings"
+	"unicode"
 
 	"gitea.maximumdirect.net/eric/distributor/internal/link"
 )
@@ -281,9 +283,15 @@ func ValidatePublishTransformPolicy(publish PublishPolicy, transform Transform)
 	if transform.MarkdownToHTML.Input != "" && !transform.MarkdownToHTML.Enabled {
 		return fmt.Errorf("transform.markdown_to_html.input requires transform.markdown_to_html.enabled to be true")
 	}
+	if transform.MarkdownToHTML.CssHref != "" && !transform.MarkdownToHTML.Enabled {
+		return fmt.Errorf("transform.markdown_to_html.css_href requires transform.markdown_to_html.enabled to be true")
+	}
 	if transform.MarkdownToHTML.Input != "" && mode != TransformModeIndex {
 		return fmt.Errorf("transform.markdown_to_html.input is only valid when mode is %s", TransformModeIndex)
 	}
+	if err := validateCSSHref(transform.MarkdownToHTML.CssHref); err != nil {
+		return fmt.Errorf("transform.markdown_to_html.css_href %w", err)
+	}
 	if transform.MarkdownToHTML.Enabled && !publish.HTML {
 		return fmt.Errorf("transform.markdown_to_html.enabled requires publish.html to be true")
 	}
@@ -293,6 +301,49 @@ func ValidatePublishTransformPolicy(publish PublishPolicy, transform Transform)
 	return nil
 }
 
+func validateCSSHref(value string) error {
+	if value == "" {
+		return nil
+	}
+	for _, character := range value {
+		if unicode.IsControl(character) || unicode.IsSpace(character) {
+			return fmt.Errorf("must not contain whitespace or control characters")
+		}
+	}
+	if strings.ContainsAny(value, "\\<>\"'") {
+		return fmt.Errorf("must not contain backslashes or HTML-sensitive characters")
+	}
+	if strings.HasPrefix(value, "//") {
+		return fmt.Errorf("must not be scheme-relative")
+	}
+	parsed, err := url.Parse(value)
+	if err != nil {
+		return fmt.Errorf("must be a valid URL reference: %w", err)
+	}
+	if parsed.Fragment != "" {
+		return fmt.Errorf("must not include a fragment")
+	}
+	if parsed.Scheme != "" {
+		if parsed.Scheme != "http" && parsed.Scheme != "https" {
+			return fmt.Errorf("scheme must be http or https")
+		}
+		if parsed.Host == "" {
+			return fmt.Errorf("host is required for absolute URLs")
+		}
+		if parsed.User != nil {
+			return fmt.Errorf("must not include userinfo")
+		}
+		return nil
+	}
+	if parsed.Host != "" {
+		return fmt.Errorf("must not be scheme-relative")
+	}
+	if parsed.Path == "" {
+		return fmt.Errorf("relative URL path is required")
+	}
+	return nil
+}
+
 func validatePathMapping(errs ValidationErrors, context string, mapping PathMapping) ValidationErrors {
 	if mapping.Mode != PathMappingPreserveRelative && mapping.Mode != PathMappingFixed {
 		errs = append(errs, context+".mode must be "+PathMappingPreserveRelative+" or "+PathMappingFixed)

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)
+			}
+		})
+	}
+}