Fix markup heading parsing, fix emphasis parsing (#36284)

Fixes #36106, fix #17958

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/htmlutil/html.go

@@ -6,6 +6,7 @@ package htmlutil
 import (
 	"fmt"
 	"html/template"
+	"io"
 	"slices"
 	"strings"
 )
@@ -31,7 +32,7 @@ func ParseSizeAndClass(defaultSize int, defaultClass string, others ...any) (int
 	return size, class
 }
 
-func HTMLFormat(s template.HTML, rawArgs ...any) template.HTML {
+func htmlFormatArgs(s template.HTML, rawArgs []any) []any {
 	if !strings.Contains(string(s), "%") || len(rawArgs) == 0 {
 		panic("HTMLFormat requires one or more arguments")
 	}
@@ -50,5 +51,35 @@ func HTMLFormat(s template.HTML, rawArgs ...any) template.HTML {
 			args[i] = template.HTMLEscapeString(fmt.Sprint(v))
 		}
 	}
-	return template.HTML(fmt.Sprintf(string(s), args...))
+	return args
+}
+
+func HTMLFormat(s template.HTML, rawArgs ...any) template.HTML {
+	return template.HTML(fmt.Sprintf(string(s), htmlFormatArgs(s, rawArgs)...))
+}
+
+func HTMLPrintf(w io.Writer, s template.HTML, rawArgs ...any) (int, error) {
+	return fmt.Fprintf(w, string(s), htmlFormatArgs(s, rawArgs)...)
+}
+
+func HTMLPrint(w io.Writer, s template.HTML) (int, error) {
+	return io.WriteString(w, string(s))
+}
+
+func HTMLPrintTag(w io.Writer, tag template.HTML, attrs map[string]string) (written int, err error) {
+	n, err := io.WriteString(w, "<"+string(tag))
+	written += n
+	if err != nil {
+		return written, err
+	}
+	for k, v := range attrs {
+		n, err = fmt.Fprintf(w, ` %s="%s"`, template.HTMLEscapeString(k), template.HTMLEscapeString(v))
+		written += n
+		if err != nil {
+			return written, err
+		}
+	}
+	n, err = io.WriteString(w, ">")
+	written += n
+	return written, err
 }