hugolib, output: Add NotAlternative to OutputFormat

To make sure CSS and similar does not appear in the AlternativeOutputFormats list.

hugolib/page_output.go

@@ -177,8 +177,6 @@ type OutputFormat struct {
 	// This is value is fetched from the output format definition.
 	// Note that for pages with only one output format,
 	// this method will always return "canonical".
-	// TODO(bep) output -- the above may not be correct for CSS etc. Figure out a way around that.
-	// TODO(bep) output -- re the above, maybe add a "alternate" filter to AlternativeOutputFormats.
 	// As an example, the AMP output format will, by default, return "amphtml".
 	//
 	// See:
@@ -222,10 +220,12 @@ func newOutputFormat(p *Page, f output.Format) *OutputFormat {
 }
 
 // OutputFormats gives the alternative output formats for this PageOutput.
+// Note that we use the term "alternative" and not "alternate" here, as it
+// does not necessarily replace the other format, it is an alternative representation.
 func (p *PageOutput) AlternativeOutputFormats() (OutputFormats, error) {
 	var o OutputFormats
 	for _, of := range p.OutputFormats() {
-		if of.f == p.outputFormat {
+		if of.f.NotAlternative || of.f == p.outputFormat {
 			continue
 		}
 		o = append(o, of)

hugolib/site_render.go

@@ -290,7 +290,7 @@ func (s *Site) renderSitemap() error {
 	n.Data["Pages"] = pages
 	n.Pages = pages
 
-	// TODO(bep) output
+	// TODO(bep) we have several of these
 	if err := page.initTargetPathDescriptor(); err != nil {
 		return err
 	}

output/outputFormat.go

@@ -63,6 +63,12 @@ type Format struct {
 
 	// Enable to ignore the global uglyURLs setting.
 	NoUgly bool
+
+	// Enable if it doesn't make sense to include this format in an alternative
+	// format listing, CSS being one good example.
+	// Note that we use the term "alternative" and not "alternate" here, as it
+	// does not necessarily replace the other format, it is an alternative representation.
+	NotAlternative bool
 }
 
 var (
@@ -93,6 +99,7 @@ var (
 		BaseName:       "styles",
 		IsPlainText:    true,
 		Rel:            "stylesheet",
+		NotAlternative: true,
 	}
 	CSVFormat = Format{
 		Name:        "CSV",